Strange thunderbird sending problem...

[Duncan Lithgow <duncan@xxxxxxxxxxxxxxxxxx>]

I've started getting this error in thunderbird, I thought I'd ask if 
anyone has seen this before...

"An error occured while sending mail. The mail server responded: unknown 
user. Please verify that your email address is correct in your Mail 
preferences and try again."

It only happend sometimes and I'm not sure what the pattern is. Anyone 
have an idea, and no it's not my Mail preference - it works fine most of 
the time.

SElinux is disabled, and setting smtp as a trusted service didn't help 
(i've set it back again)

Duncan