Complain all you want, it won't solve the problem and - here's another fact: OS X Mavericks Mail + Exchange works for many, many people.
Open the Console, clear the log, start Mail, make a message and paste a simple screen shot into it (which makes the message big enough to fail) or make a message with ~150k of text (no attachment). Whether or not there's an attachment does NOT matter. The message size is the issue. Then send the message to yourself.
Wait.
Watch the log.
You'll see 'EXCHANGE' errors appear when Mail tells you the connection failed. This is a timeout. The message file (.EML) isn't moving from the LOCAL OUTBOX to the EXCHANGE OUTBOX. This is very likely because Mailbox Behaviors is set to save Drafts locally, instead of on the server. Change that setting, re-start Mail, then try the exercise again.
I had the intermittent failures. I had the total inability to send any kind of large message. Once I made sure my account was using domain\username and Drafts were being saved to the server in Mailbox Behavior, Mail+Exchange became rock solid.
OR
Don't do anything except complain and blame software you didn't pay a dime for.