CRM Workflow Emails Not Being Sent

I hope this helps anyone who runs into a similar problem: I ran into a problem on a client's CRM server yesterday where the emails being generated from CRM's workflow rules were not being sent out, causing the workflow rule to fail. This had been working in the past, but suddenly it was not. There were a bunch of failed and paused rules in the workflow monitor. taking a look at the failure messages there revealed two different messages:
"Invalid association. the request association is not valid." and "Invalid Visibility Modification Access."

Both of these error messages, along with an error on the CRM server's event log, pointed me to look at the antivirus software. I had run into a problem once before with a CRM server that was running McAfee antivirus that was blocking outbound traffic on port 25 - the port that carries SMTP mail. This client also was running McAfee, and some googling turned up the following KB article (support.microsoft.com/kb/916925) which, among other things, suggests placing the W3wp.exe process in the antivirus software's exclusion list. (I'm not sure if this problem only occurs with McAfee, it just so happens that these are the two cases I've come across, which was helpful since it helped me narrow down the problem a little more quickly than I might have.)

The only problem was that the W3wp.exe process was already excluded from being blocked by the A/V software. In this instance, the CRM server was relaying mail to the Exchange server for outbound SMTP mail, so I thought I'd change the registry settings to make it use the SMTP service in IIS on the CRM server. Before doing this, however, the IT wiz at my client's company discovered that the AntiVirus software was also blocking the CrmWorkflowService.exe - something that's not listed in Microsoft's KB article. By adding this to the exclusion list, workflow email started flowing again.