Mailbox Enable Receiving - disables sending
Hi, When Enable Receiving in the Mailbox settings is unselected, then You can't use the mailbox to sending (auth failed).
Accroding to manual: Enable Receiving: If you don't check this box, then incoming emails for this mail account will be rejected. This makes sense if you want to use this account only for sending mail, but not for receiving.
SMTP server: POSTFIX, tested on Debian 6 and OpenSuse 12.1