This problem is encountered when you send a message bigger than 1 MB and is solved by increasing the waiting time of your software.
Thus :
Menu tools>accounts..
Select mail
Select the account in question in the list and click on properties...
Click on Advanced and move the cursor ‘expiry delay on the server’ a maximum to the right.
If the delay increase is insufficient, you can also tick the box ‘Divide the messages greater than’ and set it at 500KB.