Reference: All Fastmail Email Domains

Wondering what domain names mailbox provider Fastmail uses for their email service? You'll never get an exact list, as customers can point their own domains at the service, but it turns out that Fastmail has a built-in list of 110 email domains that they offer up for client use.

As of October 2022, those 110 domains are:

123mail.org, 150mail.com, 150ml.com, 16mail.com, 2-mail.com, 4email.net, 50mail.com, airpost.net, allmail.net, cluemail.com, elitemail.org, emailcorner.net, emailengine.net, emailengine.org, emailgroups.net, emailplus.org, emailuser.net, eml.cc, f-m.fm, fast-email.com, fast-mail.org, fastem.com, fastemailer.com, fastest.cc, fastimap.com, fastmail.cn, fastmail.co.uk, fastmail.com, fastmail.com.au, fastmail.de, fastmail.es, fastmail.fm, fastmail.fr, fastmail.im, fastmail.in, fastmail.jp, fastmail.mx, fastmail.net, fastmail.nl, fastmail.org, fastmail.se, fastmail.to, fastmail.tw, fastmail.uk, fastmailbox.net, fastmessaging.com, fea.st, fmail.co.uk, fmailbox.com, fmgirl.com, fmguy.com, ftml.net, hailmail.net, imap-mail.com, imap.cc, imapmail.org, inoutbox.com, internet-e-mail.com, internet-mail.org, internetemails.net, internetmailing.net, jetemail.net, justemail.net, letterboxes.org, mail-central.com, mail-page.com, mailas.com, mailbolt.com, mailc.net, mailcan.com, mailforce.net, mailhaven.com, mailingaddress.org, mailite.com, mailmight.com, mailnew.com, mailsent.net, mailservice.ms, mailup.net, mailworks.org, ml1.net, mm.st, myfastmail.com, mymacmail.com, nospammail.net, ownmail.net, petml.com, postinbox.com, postpro.net, proinbox.com, promessage.com, realemail.net, reallyfast.biz, reallyfast.info, rushpost.com, sent.as, sent.at, sent.com, speedpost.net, speedymail.org, ssl-mail.com, swift-mail.com, the-fastest.net, the-quickest.com, theinternetemail.com, veryfast.biz, veryspeedy.net, warpmail.net, xsmail.com, yepmail.net, your-mail.com

This list might come in handy if you're looking to segment mail to handle Fastmail subscribers separately, or if you're wanting to configure your MTA to send mail to Fastmail subscribers at a certain speed.