Was getting mail sent via relay kicked from one recipient, so tried a send to an onsite mailsever to see what the handshake was like. Came up with this:
Tue 2017-11-21 13:57:26.147: Accepting SMTP connection from 212.159.14.20:42463 to [Mailserver IP]
Tue 2017-11-21 13:57:26.350: <-- EHLO avasout08.plus.net.plus.net
Tue 2017-11-21 13:57:26.351: --> 250- Hello avasout08.plus.net.plus.net [212.159.14.20], pleased to meet you
Tue 2017-11-21 13:57:26.351: --> 250-ETRN
Tue 2017-11-21 13:57:26.351: --> 250-AUTH LOGIN CRAM-MD5 PLAIN
Tue 2017-11-21 13:57:26.351: --> 250-8BITMIME
Tue 2017-11-21 13:57:26.351: --> 250-ENHANCEDSTATUSCODES
Tue 2017-11-21 13:57:26.351: --> 250 SIZE 50000000
...
Tue 2017-11-21 13:57:26.448: Performing IP lookup (avasout08.plus.net.plus.net) <-------- !!!!
Tue 2017-11-21 13:57:26.556: * DNS server reports domain name unknown
...
Tue 2017-11-21 13:58:24.158: SMTP session terminated (Bytes in/out: 61/465)
Tue 2017-11-21 13:58:24.158: ----------
-Is that happening at Plusnet's end? I don't think recipient servers normally modify HELO strings, so I imagine it must be.
If so it would explain some messages being lost, as the more pernickety servers will barf straightaway at the sight of a nonexistent HELO domain. While others might not care.
