Tbird is a chunk of shit. Things I have tried: Mac Mail Postbox Thunderbird Airmail Mailbox Things I will not try come hell or high water: Outlook Depending on the version, Postbox/Tbird tend to be the most functional (probably because Postbox is a paid port of Tbird). If it weren't for the insane bloatware of Mac Mail and its pernicious tendency to hammer your mail server once per account every two minutes (thus forcing your reseller to consider it a DDOS attack - I shut down my wife's website before I ported to my own space), I wouldn't have ventured forth in the first place... but a Tier 3 guy told me with great confidence that only Mac Mail has the bad manners to DDOS your own server.
Have you tried Sparrow? I've used that as my primary email client for 3 years now. Even though Google bought them and it's no longer under development, it still works on Mavericks.