I can't send email on my iPhone 4S. Ever since I recently changed my password and my outgoing STMP server switched to the yahoo one I cannot receive email or send it on my iPhone. Does it have anything to do with the changing to the yahoo STMP server? On my sons iPhone he has the regular outgoing.verizon server and he doesn't have a problem with it. I have no problem at all with sending and receiving email on my computer.
The yahoo one is outgoing.yahoo.verizon.net
the regular one is outgoing.verizon.net
Also make sure the SMTP server is not using port 25. make sure it uses 465 or 587