I had the same problem and here is how I fixed it.
I had traveled out of town (away from my ISP) and just now found out that when I travel away from my ISP I need to do the following:
Go to mail preferences, click on accounts, go to outgoing mail service and click on that and edit the smtp server and click on the advanced tab. Click on use custom port and put in 587. Under authentication click on password. Now this should work the mail sending when you are away from your area (mine is roadrunner).
Now, if you are home, you should do the above, but in the custom port put 25 and under authentication click on none.
This fixed my problem. Hope it helps you.