[p2t2p]

Fastmail tells you very explicitly what to do and how to do that, I recommend doing that, at one day you may be hit with a very important email going undelivered.

[EveYoung]

I'm using the recommended Fastmail setup with my own name server, which doesn't support DMARC out of the box. Is this something I should look into to improve deliverability?

[anderspitman]

I haven't bothered with anything beyond the recommended SPF and DKIM records, and am not aware of any issues. IMO it will be years before not having DMARC is a serious issue for delivery, and I expect Fastmail to tell me when that time is getting close and/or take care of it for me.

[BenjiWiebe]

DMARC is just a TXT record. Which DNS server doesn't support TXT records out of the box?

[EveYoung]

I meant that the recommended guide doesn't include anything regarding the DMARC setup. Technically, it should be possible of course.

[AndrewDavis]

If you point to Fastmail as your name server it will even setup the records for you.

[climb_stealth]

Have you tried it with this service? My fastmail managed custom domain doesn't have DMARC set up according to this site. The other two are set.