I have identical problem. 5 IMAP accounts and 1 Exchange acount. Only on this EX account I have regularlly problems with attachements. 1 minute or 2 after, I receive the message to try again, and normally it goes.
It deppends also on the quality of the net, or the place you are at office, home, airport or hotel.
The problem is that I found today that the email is going out twice. More than one prson told me, I got your email twice.
I was 5y ago a PC/outlook user. I won't give up of Mail, and I won't come back to any other version of OS. Just wait to Apple to solve the issues. It happens to every one.
Today I had an exchange email to send with a 6MB PDF attached. I couldn't wait at lunch time, and decided just to go away, expecting to find the "send again" up on my return. Surprise! No message, and the email was simply sent by himself.
So conclusion, be cool, seat back, or just continue working, and it will go. One day this will be solved. We all know the other millions of problems with PC's... We're just too stressed. Remember the time you had to go to the post office to send your letters... :-) Or asking the operator to make you an international phone call.