On July 2nd I started getting tons of eMail bounces from various domains. These bounces are all associated with automatic notifications for topics, etc. All of them come back with the error message of "Message rejected for spam or virus content". Does anyone have any ideas on this? I'm not sure where/how to start looking for issues.

Welcome to the club.

IPB made my domain get flagged by "spam" in gmail, hotmail, you name it.

Never used the bulkmail tool ONCE! Just the normal emails that people CHOOSE to receive or not and are disabled by default.

 

Send in a support ticket with examples of rejected email.  Include the email header:

http://www.emailaddressmanager.com/tips/header.html

You might want to reduce the amount of content in your emails.  Started happening to me until I bought a plugin which limits email content.