My post “CRM Email Router: enabling logging” has raised in popularity the last few weeks. It is simple, but IMO it doesn’t don’t contain all the information needed to debug the Email Router. Hopefully, this post can give you some more information.
Locate the Microsoft.Crm.Tools.EmailAgent.xml in the folder C:\Program Files\Microsoft CRM Email\Service.
If the tags <LogLevel> and/or <LogFile> doesn’t exist, add these.
The CRM Team has a quite good post EmailAgent.xml Explained on the XML configuration file.
You have to monitor the file size of the log file. Depending on number of accounts and profiles configured, this file will grow fast. I suggest you only have have set the LogLevel to 3 during the test period (a few hours at the most), and set it back to <LogLevel>1</LogLevel> when you have performed the test scenarios needed.
Another solution is to create a command file that is scheduled. This script file should stop the email router service, zip the logfile or just delete it, and restart the email router service again.