Thanks for replying, Falco.
I love ISPConfig3 by the way.
I pretty much tried every combination of outgoing settings on Thunderbird. The one that actually attempts to connect is:
Connection Security: none
Authentication method: Password, transmitted insecurely
User name: greg@myDomain.com
(changed the domain name)
I thought about changing to use the submission port, 587, but have not enabled that yet.