Mail.app stopped working after upgrade to OS X Mavericks
I've switched to Postbox for my mail client until some of the Apple Mail glitches get worked out. Postbox seems to be built on the Thunderbird engine (which also works well) but it adds additional features. I use multiple Gmail and IMAP accounts, but I don't currently use any POP accounts -- based on past experience with Thunderbird I wouldn't expect any problems.