I had the same problem a month before and during my search I found pages
mentioning thunderbird doesn't support DIGEST-MD5.
So I have to use CRAM-MD5 and disable TLS support from Postfix and everything works without any error.
But still I dont understand,why squirrelmail could not send mail,when I
use Postfix with TLS support with any authentication mechanism.(Doesnt matter which i use authentication mechanism)
Still,I am searching page(s) about this problem.
Thanks for helped all replied.
Iyi calismalar.Basarilar...