When I have encountered this before, it has been a problem with the profile
configuration for that mailbox, likely a problem with the outgoing mailbox.
Turn your "Show details" window on (Tools/Send-Receive/Settings/Show
Progress) to see what it tells you, or go the account setup for that mailbox
and use the "Test Account Settings" button. That will tell you first if the
basic parts of Send/Receive are working correctly, and maybe the problem.