KaiMail vs Forward Email vs ImprovMX: An Honest Comparison

I built KaiMail. That is my bias declared up front, and you should take my assessment of it with appropriate scepticism. I have tried to be fair, and the comparison table below speaks for itself.

The reason I built KaiMail is the reason I can write this comparison at all. Like most developers, I collect domain names the way some people collect vinyl records. Every time a new idea lands, the first thing I do is register a domain for it. The second thing I do is put off setting up email for that domain, because setting up an email server just to receive messages is not trivial. I tried the existing forwarding services. Each one solved part of the problem. None of them solved the part I cared about most: making sure forwarded emails actually reach the inbox instead of getting flagged as spam. That gap is what drove me to build something different.

And of course there is the fact that I am unhappy that my open-source community continues to rely on things like gmail.com as their email address instead of their own domains.

So here we are. Three services that do the same core thing (forward email from your custom domain to an inbox you already use), but they make very different choices about what else to optimise for. This is my honest breakdown of all three.

What Each Service Actually Does

KaiMail is a SaaS email forwarding service that handles the forwarding problem and adds ARC and DKIM signing to improve deliverability, native webhook delivery for developers, bounce tracking, and a Japanese-language UI with JPY pricing. It is the newest and smallest of the three.

Forward Email is a source-available email service founded in 2017. Its entire codebase is on GitHub under a BUSL-1.1 licence, which means you can audit and self-host it, but the licence restricts competitive commercial use. It started as a forwarding tool and grew into a full email platform with IMAP/POP mailbox storage, SMTP sending, CalDAV calendars, and CardDAV contacts on paid plans. It is the only service here that lets you self-host the entire stack. Its user base includes Netflix Games, The Linux Foundation, and the US Naval Academy.

ImprovMX is the incumbent. Founded in 2013, it has over 200,000 users and the longest track record of the three. It offers email forwarding with regex-based routing rules, native webhook support, and a polished UI. It is the one most people have heard of.

If these three were restaurants: KaiMail is the specialist chef who does one thing (webhooks and the Japanese market) exceptionally well, Forward Email is the open kitchen where you can watch every dish being made and even cook it yourself at home, and ImprovMX is the established neighbourhood favourite that has been serving customers for over a decade.

What You Pay and What You Get

A few things stand out immediately.

Forward Email at $3/month is the cheapest way to get SMTP sending plus a full IMAP mailbox. If you want to both send and receive email with actual storage, that is hard to beat.

KaiMail PLUS at roughly $3.70/month gets you webhooks and ARC signing but no mailbox storage. You are paying for developer integration, not for an inbox.

ImprovMX's useful features start at Premium ($9/month). The Light plan at $50/year exists but is not prominently advertised, and it does not include webhooks or regex routing. If you need those features, you are paying at least $9/month.

For Japanese users, KaiMail's JPY pricing is a practical advantage. Paying in USD means dealing with foreign transaction fees on every billing cycle, and those fees add up over a year. Forward Email and ImprovMX both price exclusively in USD.

The Feature Table

Three things jump out from this table. All three services implement ARC signing, which is good news for deliverability. Forward Email is the only one with full mailbox storage. ImprovMX is the only one with regex routing. Everything else is variations on a theme.

KaiMail: Webhooks, Bounce Tracking, and the Japanese Market

Let me start with what KaiMail does not do, because honesty matters more here than marketing.

1. No regex routing. ImprovMX lets you define pattern-based rules like support-.*@yourdomain.com and route them to specific destinations. KaiMail does not have this. If you need regex routing, ImprovMX is the only option among these three.
2. No mailbox or IMAP storage. KaiMail forwards email. It does not store it. If you want a full inbox with IMAP access, Forward Email is your only option in this comparison.
3. Newer and smaller. KaiMail does not have 200,000 users or a decade-long track record. It is the newest service here, and that carries risk.
4. Restrictive free tier. The BASIC plan gives you 1 domain, 1 mailbox, 300 emails per month, and a 1 MB maximum email size. It is enough to try the service, not enough for production use.

Now, what KaiMail does well.

1. ARC signing. All three services in this comparison now implement ARC signing, which is good news for everyone. I will explain why ARC matters in the next section. KaiMail's implementation has been in place since launch, and it remains a core part of how we handle deliverability.
2. Native webhooks at a lower price point. You point KaiMail at a URL and incoming email gets POSTed there as JSON. No Workers, no API polling, no infrastructure to maintain. You can test it with Python in about five minutes. Forward Email and ImprovMX also offer webhooks, but KaiMail includes them starting from the PLUS plan at roughly $3.70/month, while ImprovMX requires the Premium plan at $9/month.
3. Bounce tracking with dynamic MAIL FROM. When a forwarded email bounces, KaiMail tracks it properly so you know what failed and why. This is more useful than it sounds once you start relying on email forwarding for anything important.
4. Japanese UI and JPY pricing. This is not a minor feature for the audience it serves. Japanese businesses and freelancers who deal with USD billing and English-only dashboards have exactly one native option here.

KaiMail is best for developers building email-to-webhook integrations, Japanese businesses and freelancers who want native-language support and local currency billing, and anyone who wants webhook delivery included at a lower price point. I wrote about why I built it here.

Forward Email: Source-Available, Privacy First, and the Only One With Mailboxes

Forward Email is the idealist of the three. Its entire codebase (frontend and backend) is on GitHub under a BUSL-1.1 licence. You can audit it, fork it, or self-host it with Docker. The licence restricts competitive commercial use, so it is source-available rather than open source by the OSI definition, but the transparency is real and verifiable.

What Forward Email does well:

1. Source-available and auditable. The only service here where you can read every line of code that handles your email. The codebase is on GitHub under a BUSL-1.1 licence, which allows auditing and self-hosting but restricts competitive commercial use. If you care about transparency, this is not a nice-to-have. It is the requirement.
2. Zero-knowledge architecture on the free tier. Forwarded emails are processed in memory and never written to disk. On paid plans, mailboxes use individually encrypted SQLite databases. This is strong privacy engineering, not just a privacy policy.
3. Full IMAP/POP mailbox storage. The only service in this comparison that gives you an actual inbox with 10 GB of encrypted storage at $3/month. If you want to stop forwarding to Gmail and start using your domain email as a proper mailbox, this is how.
4. Unlimited aliases and domains on all plans. No per-domain or per-alias charges. The free plan lets you forward unlimited aliases on unlimited domains. The limits are on sending and storage, not on how many addresses you can create.
5. Self-hostable. Since March 2025, you can run the entire Forward Email stack on your own infrastructure via Docker. Complete control, no third-party dependency.

What Forward Email does not do as well:

1. Machine-translated Japanese. The Japanese UI reads like what it is: automatically generated. It is functional, not native. If you are a Japanese user, the experience will feel rough.
2. Webhooks configured via DNS on the free plan. Forward Email does support native webhook delivery, but on the free plan webhooks are configured through DNS TXT records rather than a dashboard, which is less intuitive. Paid plans offer dashboard-based webhook configuration.
3. Greylisting. New senders are temporarily rejected with a 450 status code and retried after 5 minutes. This is an anti-spam measure, and it works, but it adds unpredictable delays to legitimate email delivery.
4. DNS-visible aliases on the free tier. Aliases are stored as public TXT records, which means anyone who queries your domain's DNS can see your email routing configuration. Privacy requires a paid plan.
5. Account bans without warning. Multiple Trustpilot reviewers have reported accounts being suspended without explanation, including for choices of domain TLD that Forward Email considers spammy. Chargebacks result in permanent bans.
6. ToS restrictions on third-party use. The terms of service prohibit using the service in a way that "benefits third parties other than your own employees and contractors." If you are embedding email forwarding into a product that serves external customers, read the ToS carefully.
7. BUSL-1.1 licence. The code is publicly available and auditable, but the Business Source Licence restricts competitive commercial use. This is not an OSI-approved open source licence. If that distinction matters to you, read the licence terms carefully.

Forward Email has a Trustpilot rating of 4.6 out of 5 from approximately 170 reviews. It is trusted by Netflix Games, The Linux Foundation, the US Naval Academy, Canonical, and University of Cambridge. That is a serious user base for a service that positions itself as the privacy-first option.

It is best for privacy advocates who want auditable code, users who need actual IMAP mailbox storage, developers who want the option to self-host, and anyone who wants the cheapest path to SMTP sending plus a full inbox.

ImprovMX: The Established Choice With Regex Routing

ImprovMX has been doing this since 2013. Over 200,000 users. A Trustpilot rating of 4.9 out of 5 from approximately 135 reviews. It is the service most people find first when they search for "email forwarding custom domain," and for good reason: it works, the UI is clean, and setup takes seconds.

What ImprovMX does well:

1. Longest track record. Over a decade of operation. That matters for email infrastructure, where reliability is earned over years, not months.
2. Regex-based routing rules. The only service in this comparison that lets you define pattern-based aliases like support-.*@yourdomain.com and route them to specific destinations. If you have complex email routing needs, this is genuinely useful.
3. Native webhook support on paid plans. Incoming email gets POSTed to your endpoint. Comparable to KaiMail's webhook delivery, though KaiMail includes it at a lower price point.
4. Polished UI. Setting up a forwarding alias on ImprovMX takes about 10 seconds. Multiple reviewers confirm this. The interface is intuitive and well-designed.
5. Generous domain limits. Up to 100 domains on the Pro plan. If you are managing email for a portfolio of domains, this scale is hard to match.
6. Permanent free tier pledge. ImprovMX has publicly committed to keeping the free plan available permanently. For hobbyists and side projects, this is reassuring.
7. High trust scores. 4.9 on Trustpilot is excellent by any standard, though the review count is relatively modest at around 135.

What ImprovMX does not do as well:

1. No Japanese-language support. English only. No JPY pricing. For Japanese users, this means foreign transaction fees and a dashboard in a second language.
2. No mailbox or IMAP storage. Like KaiMail, ImprovMX is forwarding only. If you want an inbox, you need a separate provider.
3. Spam filter false positives. There are documented cases of legitimate multipart emails being bounced by ImprovMX's SpamAssassin instance. When escalated to support, the agent incorrectly claimed the bounce was because the email was multipart, which is standard practice. This suggests a gap in email expertise on the support team.
4. Infrastructure reliability concerns. ImprovMX's own post-mortems document several incidents in late 2025 and early 2026: spam campaign overloads, API downtime, slow SQL queries from missing indexes, and delivery queue backups. These were acknowledged and fixed, but the pattern is worth noting for a service that handles your email.
5. Free tier restrictions. The free plan is limited to 1 domain and 25 aliases. Some Trustpilot reviewers have noted that the free tier feels restrictive, though ImprovMX has publicly committed to keeping it available permanently.
6. AI chatbot for first-line support. The initial support contact is an AI chatbot that has been described as unhelpful and occasionally incorrect. Escalation to human support is available, but the quality of that support has been inconsistent.
7. Acquired by a new owner in 2025. ImprovMX was acquired by Matthew Tse in January 2025. Tse has stated plans to invest in reliability and new features, and existing users were promised grandfathered pricing. However, any ownership change introduces uncertainty about long-term direction and investment priorities.

ImprovMX is best for power users managing many domains, teams needing regex routing for complex email workflows, and users who want the safety of the largest established service in this space.

Why ARC Matters (And Why Most People Do Not Know About It)

Here is the problem with email forwarding that most people never think about until their forwarded emails start landing in spam.

When someone sends an email to [email protected], their mail server authenticates it with SPF and DKIM. Your forwarding service receives it and forwards it to [email protected]. Gmail then checks the authentication again, but now the email came from the forwarding service's server, not the original sender's. SPF fails because the forwarder is not in the sender's SPF record. DKIM might still pass if the message body and signed headers are unchanged, but if the forwarder modifies the message (adding headers, altering content), DKIM breaks as well. When both SPF and DKIM alignment fail, DMARC rejects the email.

ARC (Authenticated Received Chain) solves this by letting the forwarding server cryptographically vouch for the authentication it saw when the email arrived. The receiving server can then verify that the email was legitimately authenticated at the previous hop, even if SPF and DKIM do not align at the final destination.

Google and Yahoo tightened DMARC enforcement starting in 2024. This is not a future risk. It is a present reality. Forwarded emails without ARC are increasingly being rejected or sent to spam by these providers.

All three services in this comparison now implement ARC signing. This is good news for users. KaiMail has had ARC signing since launch, Forward Email deployed it in 2020, and ImprovMX has also added ARC signing to forwarded emails. The important thing is that whichever service you choose, ARC is there to help protect your forwarded email deliverability.

An honest caveat: if your forwarded emails are currently reaching inboxes without problems, you may not need to think about ARC. But the trend is toward stricter enforcement, not looser. Having ARC in place is a safeguard, and it is reassuring that all three services now offer it.

So Which One Should You Use

This is the part where I am supposed to say "it depends" and then dodge the question. I will not do that.

1. If you are a developer building email-to-webhook integrations and you want incoming email POSTed to your application without writing infrastructure code: KaiMail or Forward Email. Both offer native webhook delivery. KaiMail includes webhooks starting from PLUS (roughly $3.70/month). Forward Email offers webhooks on all plans including the free tier (configured via DNS TXT records on the free plan, via dashboard on paid plans). ImprovMX also offers webhooks, but only on Premium ($9/month) and above.

2. If you are a privacy advocate who wants to audit every line of code, or you need full IMAP mailbox storage, or you want to self-host your email infrastructure: Forward Email. It is the only option here that checks those boxes, and at $3/month for sending plus 10 GB of storage, the value is hard to argue with. Note that the licence is BUSL-1.1, which restricts competitive commercial use.

3. If you manage many domains and need regex routing for complex email workflows, or you want the safety of the largest user base and longest track record: ImprovMX. Its regex routing is genuinely useful, and 200,000 users over a decade is a real signal.

4. If you are a Japanese business or freelancer who needs native-language support and JPY pricing: KaiMail. The other two services either offer machine-translated Japanese (Forward Email) or no Japanese at all (ImprovMX). Currency conversion fees on USD billing are not trivial over a year.

5. If you care about forwarded email deliverability (ARC signing): Good news. All three services now implement ARC signing. KaiMail has had it since launch, Forward Email deployed it in 2020, and ImprovMX has also added it. Whichever you choose, ARC protection is included.

6. If you want the cheapest way to get SMTP sending plus a full mailbox: Forward Email Enhanced at $3/month. No one else comes close for that combination.

7. If you want the most generous free tier for simple forwarding: Forward Email. Unlimited aliases and unlimited domains. Just be aware of the DNS-visible aliases and TLD restrictions on the free plan.

Not every use case needs KaiMail. If you want IMAP storage, Forward Email is the answer. If you need regex routing, ImprovMX is the answer. If you want webhooks on the free tier, Forward Email is the answer. The comparison table does not lie.

The Comparison Table Does Not Lie

I built KaiMail because I believed ARC signing and native webhooks mattered for forwarded email. I still believe that. I am also glad that Forward Email and ImprovMX now implement ARC signing, because better deliverability for forwarded email benefits everyone. Forward Email's source-available transparency and ImprovMX's regex routing and decade of reliability are real strengths that serve real users.

The right choice depends on what you need. Pick the thing that solves your problem.

The comparison data and information in this post is based on independent research in April 2026 by LaLoka Labs LLC, which runs KaiMail. We believe that this post is correct at the time of publication.