You don't say whether you're using Dovecot/Maildir or the old mbox on your DirectAdmin server.
If you're using DirectAdmin with Dovecot/Maildir, then Plaintext Auth should just work; it's implemented in the standard SpamBlocker exim.conf 2.x and 3-beta files.
Your ISP could be blocking outgoing port 25 traffic; many do. You might want to try telling Outlook to use port 587 for outgoing emails through your server; that's also implemented in DirectAdmin's exim.conf files.
Jeff