This article was originally published on a (now defunct) personal blog. It was reposted here in 2024.

It's not hard for a mail server to forward emails to a different recipient, not difficult at all. In fact, it's the kind of thing that mail servers are designed to do, which makes it almost trivial. Cool, you think, forwarding email for my personal domain should be easy.

And you would be correct to think so, at least for the forwarding part. The real question is whether the forwarded message is actually accepted by and delivered to the final recipient.

That's where things get interesting, but only if you're paying very close attention.

Since you know what you're doing, you don't just naïvely forward your domain to Gmail and call it a day. This would ignore modern best practices. Instead, you do your homework and learn that forwarding to providers like Gmail requires extra care.

You read up on SPF, DKIM and DMARC. You quickly realize that proper forwarding means deploying Sender Rewriting Scheme (SRS) and Authenticated Received Chain (ARC), which you diligently set up.

Whether you configure everything yourself or rely on your registrar or hosting provider doesn’t really matter. The outcome is the same: email sent to your custom domain gets forwarded to Gmail, and it appears to work.

But does it, really?

From years of experience running mail servers to forward to large email providers such as Gmail, I can say this with confidence: most forwarded emails are delivered. In practice, based on years of running such setups, well over 99% of emails arrive without issue.

In other words: most, but not all.

And somehow, the mails that don't make it through aren't random spam. They're often among the most important email messages you receive. Worse, they're frequently sent by some automated process, so no one notices when they bounce. Not the sender, and not even you, not unless you are explicitly expecting such email and realize it's not coming through.

Surely, you think, there must be technical solution to this conundrum.

Alas, there isn't.

The issue isn't with your forwarding setup. The issue is that today's email authentication protocols are fundamentally ill-suited to forwarding emails. Specifically, successful forwarding depends on the original sender signing their email messages with DKIM and using a fairly relaxed DMARC policy.

In other words, much of this is outside your control.

There is no simple fix. You are largely at the mercy of:

• The sender's mail configuration and DMARC policy
• The recipient's spam filtering decisions

You can resend the rejected email messages as an attachment. When implemented carefully, this works reasonably well with major providers. Both Gmail and Yahoo will even display these almost as if they were the original emails.

It's effective, but it's also kind of an ugly hack, and not something that is easy to implement out of the box.

Postdirect guarantees the delivery of forwarded emails to Gmail, solving the above troublesome issue, by using the Gmail API to deliver emails that are being rejected by Gmail over SMTP.