I'm managing a corporate Email server for nlscan.com serving email@example.com. However, for historical reason, we still support firstname.lastname@example.org. That is, email@example.com and firstname.lastname@example.org refer to exactly the same mailbox and mail account. We recently find that some alien domain like @corp.url.com.tw fails to send email to us; our SMTP log shows that @corp.url.com.tw's SMTP server issues SMTP command
We do not recognize nlsgate.nlscan.com as recipient address, so it fails. We just allow 'RCPT TO: ＜email@example.com＞' .
Where does that @nlsgate.nlscan.com come from? I fact, on our DNS, mail.nlscan.com is defined to be a CNAME of nlsgate.nlscan.com .
If sending from Gmail or Hotmail, we see the correct
So, I'd like to ask, is the behavior of corp.url.com.tw's SMTP server RFC compliant? If it is compliant, we have to make mail.nlscan.com a A record instead of a CNAME record, right? And please tell me which RFC is it?