Strange e-mail error message (Thunderbird)

I'm not sure if this is the right area to put this in. I have a friend who uses Thunderbird (I have no idea if the problem is specific to that program) and some times when she sends mail she gets the error message below. She has multiple accounts that she's accessing and using via Thunderbird. Any idea what's wrong and/or how to fix it? From our end everything does seem to be setup correctly...

"An error occured while sending mail. The mail server responded: 5.7.0 Must issue a STARTTLS command first. 6sm164427ywp.24. Please verify that your email address is correct in your Mail preferences and try again."

See here: http://forums.mozillazine.org/viewto...st=0&sk=t&sd=a