You are reading this because one of your autoresponder (either a bounce message or a vacation message) was sent back to us to the header sender of one of our messages and not the envelope sender. The envelope sender is that is given in "MAIL FROM: " during the SMTP transaction, while the header sender is what is in the "From: " field inside the Email itself. Answers from people go to the header sender (the address you see in the Email) while answers from machines (autoreplies, vacation messages, bounces, etc...) should go to the envelope sender (actually bounces MUST) Checking if the receipient in the headers of a mail before you trigger an autoreply is a good thing and you must check for "Precedence: bulk" or "Precedence: list" and not send autoresponder messages if you see these headers (mailman password reminder mails use it). However, unless your mail server prevents your autoresponder from doing it, you should also reply to the envelope from of the original message and set your header from to <>, as well as set the "Precedence: bulk" header. The rationale is that you should do as much as you can do avoid loops, and send automated responses, whether they are bounces or autoresponses, to the envelope from. It helps the receipient sort automated replies from actual user answers (in some cases it's very useful if you need to autoprocess autoreplies with a program and send user answers to an actual admin) By sending bounces/autoreplies to the header From, you are: - A risk for a bounce storm between mail servers - Preventing auto unsubscribe or auto discard programs from working - Adding to the workload of already overworked sysadmins. Your bounces should also follow those rules: - send autoreplies as bounce messages (envelope sender set to <>) - never reply to bounce messages (envelope sender set to <>) In addition, if you have a vacation autoresponder, it should also follow those rules (list courtesy of Nigel Metheringham): - not reply to list mail (see Precedence or list headers) - not reply to autoreplies (use headers to detect this) - rate limit autoreplies - eg one per address per week - reply only to personal mail (ie mention one of your addresses in to/cc) If the documentation for your program doesn't outline how to make the change, you can't get technical support for it, and/or it just isn't possible, you should change the said program. While we try not to do this, if you can't change your configuration, and it disrupts our servers too much, we will have to block all your mail. Thank you for your cooperation.