@jeffrey6f729b2968bb9ab9eaf

For A entries, in most cases, with most providers, you only need to specify mail and not the FQDN, or fully qualified domain name.

So instead of:

A          mail.yourdomain.com          IP_ADDRESS

… you’d want to use:

A          mail          IP_ADDRESS

This is the case with DigitalOcean’s DNS, as well as most domain registrars. Your domain is appended by default, so there’s no need to add it with the entry.