Yesterday I ran into a problem while sending an email with my iPhone which was a bit too large (>300 mb… 
)
I kept trying to send the mail in the meanwhile slowing down my phone. I took some time figuring out how to solve this. The solution was simple, enable flight mode on the (i)phone and try sending the mail again (retry). Now it stops direct and gives and error. Now you have the option to select the outbox folder and remove the large mail.
Somehow the solution found on the internet (holding down the home button) didn’t seem to work for me…