Not unique to Fastmail. Any domain that sends email should have DKIM/SPF/DMARC. SPF is quickly becoming irrelevant, but it is an easy configuration item.

Recommend mxtoolbox for validating configurations https://mxtoolbox.com/

Specifically send a test email to ping@tools.mxtoolbox.com and it will advise you of your current settings.

Dmarcian has good resources on DMARC specifically, and can act as an RUA report reader as a paid service. https://dmarcian.com/alignment/

When you manage your domains through Fastmail it does it automatically. I certainly haven't had to configure it myself.

There is a neat website to check your email settings that was on the HN front page earlier this year:

https://www.learndmarc.com/

In general, never have an MX configured in your authoritative DNS zonefile without proper SPF and DKIM. Deliverability to outbound SMTP destinations will be very poor.

Not fastmail specific.

This is true of any mail service and is not at all unique to Fastmail.

Also add DMARC to the list aswell, and make sure to warm up the domain again once you're done.

Agreed, and thank you for the PSA.

This hit me a week ago. After a friend let me know, configuring DKIM/SPF did the trick in minutes.

I added DKIM/SPF over a year ago, and they still sometimes get flagged. It doesn't help that one sad/spiteful person marked it as spam in his Yahoo account (at least I was notified that someone did, and because I'm not actually a spammer, I quickly figured out who it was).

I have a custom domain on shared hosting, and apparently the hoster does not support DKIM.

Is that really bad?

I sent a mail from it to a gmail account, and it was not flagged as spam

Also, when I send a mail from my university address, it says, the DKIM user identifier does not match the from header

This happened to me, starting a few days ago. It appears to be fixed after adding the DKIM/SPF records.

I know nothing about DKIM/SPF. Is there a reason this only applies to older Fastmail users?

I noticed this recent change too. Deliberate degradation of service via competitors (Fastmail is objectively not a spam relay and I'm sure the folks at GMail know that) is just more fodder for the coming anti-trust case.

thanks