Configuring DNS for Fastmail subdomain addressing at Namecheap

06 May, 2022

I recently signed up for Fastmail to host my personal email.

One feature that attracted me to the service is their subdomain addressing. Briefly, this allows you to provide unique or "disposable" email address to different services while slightly obfuscating your actual email address. Unique, per-service addresses helps when creating mail rules to automatically process email, and in the case that you need to block mails to a certain address that is being spammed.¹

I've configured custom domains so that I can use my own domain name(s) to receive mail. The subdomain addressing feature works with custom domains, but Fastmail's domain setup guide did not make it clear which DNS entries I needed to create.

When creating MX records, a host value of @ means the domain itself. This was the only recommended host value in the documentation Fastmail provides for Namecheap, and the setup wizard reported everything was set up correctly.

However, subdomain addressing did not work. I found their manual DNS configuration guide, and noticed it mentions:

Subdomain Mail

Allows you to receive email at subdomain addresses, e.g. foo@user.{mydomain.com}. MX *.{mydomain.com} 10 in1-smtp.messagingengine.com MX *.{mydomain.com} 20 in2-smtp.messagingengine.com

In addition to the default or "@" MX records, you must also create wildcard records for the host *. This will allow any subdomain, e.g. example.domain.tld, to properly route to the mail exchanger.

I have these four entries on each custom domain I'm pointing to Fastmail:

Everything is working great! I confirmed with Fastmail Support that this is indeed the correct configuration.

This should have been obvious to me, but I guess I was following the instructions too closely instead of my instincts.