One of my favorite features of Fastmail is mail routing. I can use that to create any number of ad hoc email addresses in the format of anything at jamie.thingelstad.com. I’ve been using this feature for a while and to set it up you create a wildcard MX entry for your domain pointing to Fastmail. This way it works for all users of your domain. A couple of weeks ago these addresses stopped working though and I had no idea why.

I actually raised the issue with Fastmail support and they confirmed that there was no MX entry for jamie.thingelstad.com. They then asked if I had setup any CNAMEs or A records recently for that name. Then it hit me, I had recently created a TXT entry for jamie.thingelstad.com.

It turns out the second that there is any record of any type on a name you no longer get the benefit of the wildcard entry. I created the specific MX entries for jamie.thingelstad.com and everything started to work just as it should.