Mail.app stopped working after upgrade to OS X Mavericks Mail.app is not working since the upgrade to OS X Mavericks. I read that I should try to uncheck SSL. However it won't let me. All my emails just sit in the outbox. Is there a way to remedy this or is there another mail software I can download?

I'm using Gmail which I know has types of issues and have accepted that but it's my business POP emails that is frustrating me.
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.