Using Thunderbird to connect to the Exchange server, I was getting the error: 5.5.4 Invalid Address. Turns out it's something to do with Exchange not liking the EHLO parameter generated by Thunderbird. You can override this value with the following entry in your prefs.js file (change 'smtp1' to whichever is the smtp server you need to fix):
user_pref("mail.smtpserver.smtp1.hello_argument", "your_hostname")
