I recently setup an exchanger server for a customer. We were getting returned e-mail for about a day due to a reverse DNS lookup problem That has been resolved but there are a couple of mail servers the keep returning our e-mail. the most common reson is below. When i google it I get so many variation that it is hard do decipher.
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<mail.xxxx.org #5.5.0 smtp;554 Transaction Failed Spam Message not queued.>