Must your mail be sent to mail server, do you not have direct connection to Internet? Why not send direct? If you have to go thru a path of SMTP servers, I have no solution.
The SMTP server was off line several hours & several hundred miles away plus I wanted to try Kontact for email. Gave Kontact my email address, pop3 server address for incoming mail and selected Sendmail rather than giving the SMTP server address. My Kubuntu lappy is now it's own mail transport agent; the SMTP server is not needed now. No further need for a outgoing server to help or hinder mail sending.
If, when I go back to Thunderbird, it will use Sendmail for quicker sending.
Exim4 in default MTA for Kubuntu8 but Sendmail was default selection of Kontact, so Sendmail is in use now.