Mac OS X 10.9.x ? (have to check)
Using Yahoo with Apple's Mail program.
On Friday, there was a .PDF attachment of about 2.5 MB that just couldn't be sent. When I looked at the Activity window, I saw that the message would not go past the 46% sent indicator.
I restarted the Mail program, and used MainMenu Pro to "Vacuum Apple Mail Index"
It still wouldn't go past 46%
Any known reason why this occurred?