host yy.com [XXX.XXX.XXX.XXX] said: 550-Verification failed for 550-No such person at this address” 550 Sender verify failed (in reply to RCPT TO command)

Well that’s something you don’t see every day! Definitely worth mentioning and writing done, since I may refer to this story to my IT grandchildren….. A customer of ours, suppose xx.com sends an email to yy.com The user username@xx.com gets a non-deliverable email from our on-premises Mail server (NDR) containing the following text: host yy.com…