Apple has a known bug in their mail client. It was actually introduced when they went from 10.4 to 10.5 on the mac computer, and it was carried over into their mobile devices as well. I used to beta test for apple and actually reported that bug. I was told at the time, (It's only happening to a few people so it must be a problem with your settings, and not the software). I and many others quit beta testing at that point, since we knew otherwise and felt we were being dismissed. "Cannot send email on mac" 199 Million hits on google, and "Cannot send email on ipad" yields 76 million hits. Here is what you can try. MAKE SURE you are using IMAP, so your email doesn't get deleted. Then delete the account from your mac, and then recreate. That's the advice of APPLE, btw. What you can TRY to do is first, delete ALL email in the outbox, then go into the configuration for the mail client. Go into server settings, Under SMTP uncheck the "Automatically manage connection settings", Change the port from 587 to 465, then Delete and re-add the password, and then save the settings by clicking on save int he bottom right of the server settings column. ALSO, ensure no email hung in outbox on any other devices you use to send/receive mail.