I'm hoping someone has encountered this weird behavior or something like it before and can point me down a path, because all my research has turned up nothing so far.
I had an email account recently get breached due to a re-used password, and that account was used to send a bunch of spam out from a server I help manage. We changed the password on the account as soon as we found it happening and the outbound flood stopped. Shortly after that, however, I started seeing a very, very strange behavior. Sometimes, and I haven't yet been able to identify the trigger or pattern, when users on this server send email to a forward that contains around 50 or so email addresses (they use it like a private distribution list) they will get anywhere from 1-10 bounces from Gmail. Not every email sent to the forward has this happen, and not even every email from a particular user. The outbound spamming caused the server's reputation to go in the tank with Google, and if it weren't for that, I wouldn't know this was happening, because they get the bounces from Gmail accounts that absolutely ARE NOT in the forward or part of the email chain AT ALL. I'm kind of freaking out here because while I haven't found a breach of the actual server / OS, this feels like someone has been able to inject something somewhere into my server that I simply can't find. It is especially troubling because a user who is not on this domain, but is part of the group and therefore uses the forward from time to time, sent something to the forward today and got Gmail bounces. I don't see anything in the send log that shows the server even trying to send to Gmail, which only adds to the ghost story. Any ideas, paths to go down, anything would be greatly appreciated here. I'm about to just rebuild the whole thing from scratch on a new VM, but if I'm overlooking something simple don't want to put the users through that. Thanks in advance. Chas
