It got rid of a few bugs (considering your avatar that isn't a real issue for you) like fixing the bug the that stopped 4.0.28.1 from working with Exchange, a real show stopper according to those that were using that as a server.
But here's everything:
What's new in 4.0.28.3 since 4.0.28.2:
[-] (#0005795) Import option of 'Run external action' filter action broken
[-] (#0006167) Voyager is adding redundant lines to global.ini
[*] glyphs for trash in folder tree replaced
[*] icons fixed for address book quicksearch popup and memo popup menus
[-] (#0007194) Folder|Check for viruses command was disabled until a new anti-virus plug-in was added
[*] (#0006632) Folders with all messages read should not change their position in the list of the unread Folders tab [before moving to the next folder]
What's new in 4.0.28.2 since 4.0.28.1:
[*] glyphs for trash, copy, cut icons changed
[*] Since this version, the option to use the "X-Mailer" header is turned OFF, even if you had it ON in a previous version of The Bat! You can go to the Options/Preferences and turn it ON again, but we don't recommend it.
[*] icons added into address book/quick search popup menu
[-] There were temp files left after importing messages from other mail clients.
[-] (#0007187) Using MS Exchange (MAPI) messages stay on the outbox without being sent
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I'm just a user of The Bat! I don't work for Ritlabs.