We have an Exchange 2003 server that had several users who were sending every message out with a read receipt, delivery receipt and high importance. At least one of the blacklists we requested to be delisted from had these items listed as a potential reason for being added.
I have done my best to verify security on the Exchange 2003 server, but would like to solicit any advice for making sure there are no features or settings active that may be causing problems. One issue that occurred was when we configured an external user to have POP3 access, a flood of SPAM starting coming through a few days later. We have since turned that off and the SPAM has stopped, but I would still like to make sure things are safe and if possible be able to use POP3 access as well without compromising security.
Any advice will be greatly appreciated.