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.
A side issue here is that if you don't have an MX record configured (say, you figured a domain isn't used for mail), it doesn't mean "we don't accept mail". You'll be surprised at how much spam ends up being directed at your apex A record, because according to the RFC that's where it goes in the absence of an MX record. Use
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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.
A side issue here is that if you don't have an MX record configured (say, you figured a domain isn't used for mail), it doesn't mean "we don't accept mail". You'll be surprised at how much spam ends up being directed at your apex A record, because according to the RFC that's where it goes in the absence of an MX record. Use
MX 0 .
For such domains.