Q: can receive mail but stopped being able to send mail

I can receive mail on my MacBook Pro, but, after 3:03PM on 9/26/2014 I stopped being able to send mail. I get the follingg dialog box.

Interestingly enough the data in this dialog box is out of date. My ISP at this location is not Verizon and there is no mention of iCloud. Nonetheless mail has worked without incident at this location for two years. At my office where I have a MacPro (4.1), Verizon is the ISP but, I believe, iCloud is the other listed server. (Im not there right now, so I ca't check). Incoming and outgoing mail work perfectly at that location. Both computers are rurnning OS 10.8.5. I'd appreciate any insight anyone might Have,

Thanks,

Stephen