Email Forwarding Services Compared: KaiMail, Forward Email, and ImprovMX

This article was rewritten on 2026-09-17 to reflect current pricing, feature changes, and our own expanded service offerings since the original publication in 14th April 2026. The slug and URL remain unchanged.

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.

There are other email forwarding services, of course. 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. The three services do the same core job: forward email from your custom domain to an inbox you already use. They make very different choices about what else to optimise for. This is my honest breakdown of all three.

What Each Service Does

KaiMail is a SaaS email forwarding service. Beyond forwarding, it adds ARC and DKIM signing for deliverability, native webhook delivery, bounce tracking, SMTP and HTTP email sending, mailing lists, 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. You can audit it and self-host it, but the licence restricts competitive commercial use.

It started as a forwarding tool and grew into a full email platform. Paid plans add IMAP/POP mailbox storage, SMTP sending, CalDAV calendars, and CardDAV contacts. It is the only service here that lets you self-host the entire stack.

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 routing rules, native webhook support, and a polished UI.

If these three were restaurants, KaiMail would be the specialist chef who does one thing exceptionally well. Forward Email would be the open kitchen, where you can watch every dish being made. ImprovMX would be the neighbourhood place that has kept customers for over a decade.

What You Pay and What You Get

A few things stand out.

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 exists but is not prominently advertised, and it does not include webhooks or regex routing.

For Japanese users, KaiMail's JPY pricing is a practical advantage. Paying in USD means dealing with foreign transaction fees on every billing cycle.

The Full Feature Comparison

Three differences matter most. All three services implement ARC signing, so deliverability should be comparable. Full mailbox storage is unique to Forward Email. Regex routing is unique to ImprovMX.

KaiMail: Webhooks, Sending APIs, and the Japanese Market

The weaknesses come first, because honesty matters more than marketing here.

No regex routing. ImprovMX lets you define pattern-based rules and route them to specific destinations. KaiMail does not have this. If you need regex routing, ImprovMX is the only option among these three.

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.

Newer and smaller. KaiMail does not have 200,000 users or a decade of history. It is the newest service here, and that carries its own risk.

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.

What KaiMail does well:

ARC signing. ARC signing is central to how KaiMail handles deliverability. It has been part of the service from the start.

Native webhooks at a lower price point. You point KaiMail at a URL and incoming email gets POSTed there as JSON. There is no Worker to deploy, no API to poll and no infrastructure to maintain. Forward Email and ImprovMX also offer webhooks. KaiMail includes them from the PLUS plan at roughly $3.70/month. ImprovMX requires the Premium plan at $9/month.

Bounce tracking with dynamic MAIL FROM. When a forwarded email bounces, KaiMail tracks it properly so you know what failed and why.

SMTP sending. Standard authenticated SMTP on port 587 with per-domain DKIM signing. Works with any email client (Thunderbird, Apple Mail, Outlook), with no custom API and no vendor lock-in.

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.

HTTP sending API. A JSON API that lets you send email over HTTPS using the same credentials as SMTP. Useful when port 587 is blocked, which happens in serverless environments and behind corporate firewalls.

Mailing lists. Public, broadcast, and discussion lists that run on your custom domain, with proper DKIM signing and DMARC alignment. Includes public subscribe pages with double opt-in. Neither Forward Email nor ImprovMX offer this.

Community upgrade programme. Open source projects and volunteer groups can apply for free upgrades to paid plan features.

KaiMail suits five groups:

• Developers building email-to-webhook integrations.
• Japanese businesses and freelancers who want native-language support and local currency billing.
• Teams that want webhook delivery at a lower price point.
• Community organisers who need mailing lists on their own domain.
• Open source maintainers who want professional email on no budget.

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

Forward Email is the idealist of the three. Its entire codebase is on GitHub under a BUSL-1.1 licence. You can audit it, fork it, or self-host it with Docker.

What Forward Email does well:

Source-available and auditable. Forward Email is the only service here where you can read every line of code that handles your email.

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.

Full IMAP/POP mailbox storage. This is the only service in the comparison with an actual inbox. You get 10 GB of encrypted storage for $3/month.

Unlimited aliases and domains on all plans. Forward Email charges nothing per domain or per alias. The free plan lets you forward unlimited aliases on unlimited domains.

Self-hostable. Since March 2025, you can run the entire Forward Email stack on your own infrastructure via Docker.

What Forward Email does not do as well:

Machine-translated Japanese. The Japanese UI reads like what it is: automatically generated. It is functional, not native.

Greylisting. New senders are temporarily rejected with a 450 status code and retried after 5 minutes. This works as an anti-spam measure, but it adds unpredictable delays.

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.

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, and the US Naval Academy.

Forward Email suits four groups:

• Privacy advocates who want auditable code.
• Users who need actual IMAP mailbox storage.
• Developers who want the option to self-host.
• 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. It has over 200,000 users and a Trustpilot rating of 4.9 out of 5 from approximately 135 reviews.

What ImprovMX does well:

Longest track record. ImprovMX has operated for over a decade. Reliability in email infrastructure is earned over years, not months.

Regex-based routing rules. ImprovMX is the only service here that lets you define pattern-based aliases and route them to specific destinations.

Native webhook support on paid plans. It works much like KaiMail's webhook delivery.

Polished UI. Setting up a forwarding alias on ImprovMX takes about 10 seconds.

Permanent free tier pledge. ImprovMX has publicly committed to keeping the free plan available permanently.

What ImprovMX does not do as well:

No Japanese-language support. English only. No JPY pricing.

No mailbox or IMAP storage. Like KaiMail, ImprovMX is forwarding only.

Spam filter false positives. There are documented cases of legitimate multipart emails being bounced by ImprovMX's SpamAssassin instance.

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, and delivery queue backups.

Acquired by a new owner in 2025. ImprovMX was acquired by Matthew Tse in January 2025. Any ownership change introduces uncertainty about long-term direction.

ImprovMX suits three groups:

• Power users managing 30 or more domains.
• Teams that need regex routing for complex email workflows.
• 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)

Email forwarding has a problem most people never think about, until their forwarded mail starts 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 passes it on to your Gmail.

Gmail checks the authentication again. This time 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 body and signed headers are untouched. If the forwarder modifies the message, DKIM breaks too. When both fail, DMARC rejects the email.

ARC (Authenticated Received Chain) solves this. The forwarding server cryptographically vouches for the authentication it saw when the email arrived. Google and Yahoo tightened DMARC enforcement starting in 2024. Forwarded emails without ARC are increasingly being rejected or sent to spam.

All three services in this comparison now implement ARC signing. Forward Email deployed it in 2020, ImprovMX added it, and KaiMail has had it since launch.

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.

If you are a developer building email-to-webhook integrations: KaiMail or Forward Email. Both offer native webhook delivery. KaiMail includes webhooks from PLUS (roughly $3.70/month). Forward Email offers them on all plans, including the free tier.

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: Forward Email. It is the only option here that checks those boxes.

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

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).

If you want the cheapest way to get SMTP sending plus a full mailbox: Forward Email Enhanced at $3/month. Nothing else here offers both at that price.

If you need mailing lists on your own domain: KaiMail. Public, broadcast and discussion lists are available from the PLUS plan. Neither of the other two offers them.

If you run an open source project or a volunteer group: KaiMail. The community upgrade programme gives free upgrades to paid plan features.

If you want the most generous free tier for simple forwarding: Forward Email. Unlimited aliases and unlimited domains.

Not every use case needs KaiMail. If you want IMAP storage, Forward Email is the answer. If you need regex routing, it is ImprovMX. Pick the one that solves your problem and ignore the rest.

The comparison data and information in this post is based on independent research in September 2026 by Kafkai Giken, Inc., which runs KaiMail. We believe that this post is correct at the time of publication.