I added some new IP addresses and domains to a VPS (Centos) managed with Vesta Control Panel. Everything seemed to work, but then, a few hours later, I noticed some emails (SMTP) from a previous, unchanged domain on the same VPS were bouncing to some addresses but not others.
Investigating, I looked in the DNS records for the older (in theory unchanged) domain and saw some unfamiliar entries. I certainly didn't add these myself.
I can't find anything helpful on "mail._domainkey" online (and most of the few, uninformative, hits I did find were in Russian). I'm wondering if these could be the by-product of some Vesta feature I was unaware of, or (hopefully not) some symptom of an attempted attack:
mail._domainkey TXT "k=rsa; p=MIGfMA0GCShPSdoV5Ynvcb+OAEXNkYfu3A739VBNKPNGiEjKyqGSIb3DQEBAQD3AcJa3UEHUAA4GNADCBiQKBgbKJB38x9E8ORC6I3CXbqt5P0wmX4d216O6faEG96uWO0NpoOO4A2qLNBqf6lqCgQDEkCtpZfRLhSL36BAAZSOeuCtXr30PlIDXwzhdZJ3wVFObgFF568lTYfgyiwIDAQAB" @ TXT "v=spf1 a mx ip4:184.108.40.206 ?all" _domainkey TXT "t=y; o=~;"
I mixed up the long string of characters in case it was a password hash or similar and I anonymised the IP address but otherwise these are unchanged.
mail._domainkey? What could these DNS entries mean?
Possibly relevant context: the domain these rules apply to is also used for private nameservers used by my new domains. https://mxtoolbox.com reports "SMTP Reverse DNS Mismatch" on the domain, which I was working to fix when I found these strange entries, but no other warnings or errors beyond slow SMTP connection times.