Maybe it is some kind of anti-Spam messure. Maybe they reject bulk
email or there was something in the content of your mail, which
triggered this.
ASSP e.g. can be configured to reply 5xx to Spam Messages, so the
sender will think the adress is invalid. But normal content may pass.
(If this is a
Subject: Re[2]: [IMail Forum] Understanding Returned Mail
Maybe it is some kind of anti-Spam messure. Maybe they reject bulk
email or there was something in the content of your mail, which
triggered this.
ASSP e.g. can be configured to reply 5xx to Spam Messages, so the
sender will think
