We recently upgraded from an old server running Fedora Core 1/ Plesk 7.0.1 to a new box running FC3/ Plesk 7.5 reloaded. We are having some strange problems with the email on the new server. Some emails are getting to the server recipients, while others are just sitting on the server. I can't find any rhyme or reason as to why some emails stay on the box, while others make it through no problem. Outlook is not giving me any error messages, it's just ignoring certain emails. The emails are viewable via webmail (Horde). We are running Qmail with Spam Assassin enabled, and POP3. Where the heck can I start to check?