I'm experiencing yet another type of problem...
I have Mavericks Mail with two accounts, one talking to Exchange Server 2010 and another with POP mail for my personal ISP account. Mail delivery from Exchange is slow (as others have reported), and when sending a mail item to Exchange, the Mail app often asks me to "Retry with the selected mail server". It then lets me choose between the Exchange and POP/ISP mail servers. If I select the Exchange server and click retry, the mail item gets sent very quickly.
The retry request doesn't appear to be associated with the size of the mail being sent, nor to how long I've been connected to Exchange, etc. It seems to be random. It's a nuisance, but at least it send each item when I retry as requested.