Failure to send outgoing mail to an SMTP server is an occasional problem which can be tricky to solve. Here are some clues, and a magic solution.
SMTP
Messages arriving in Mail from my ISP keep arriving in duplicate. Why?
Since updating Yosemite, I cannot send any mail out. Why not?
Most common problems are failure to collect or send, but this gives you clues to solving many other problems such as stuck messages.
Electronic mail remains one of the fundamental network services. Here I explain POP, IMAP, and SMTP basics.
The Eclectic Light Company 