Found a strange thing today with postfix.
Turns out that if you have the current network you are on (ex. 192.168.0.0/24) listed in your "mynetworks" line in your postfix "main.cf" It will not show the "AUTH PLAIN LOGIN" when you telnet to the local server port 25 and issue an ehlo localhost.
This can cause email clients on your local network to act funny and claim that the mail server dose not support SMTP authentication.
It seems for now at least specifying servers trying to relay on a "per IP" basis is probably best practices anyway.
Thought this might assist someone. The error Thunderbird spit out was something like this. "
"Sending of message failed."
"An error occurred sending mail: Unable to authenticate to SMTP server xxxxxx.xxx. It dose not support authentication (SMTP-AUTH) but you have chosen to use authentication. Uncheck 'Use name and password' for that server or contact your service provider."