Hmmm.   Our server is blacklisted by AOL/Yahoo due to "user complaints."  I need to find which domain is generating the emails that keep us on the blacklist. (We are clean everywhere else.)  Any ideas how I can search the logs for this?  (Maybe using a 3rd party app like notepad++ to open the log file?)

Andrew Notepad++ 64bit specifically, is really good for opening large log files.
What you'll wanna do is try starting with the emailaddress and a time frame, maybe find in the logs the communication to AOL/Yahoo, collect some of these IP's. You can try searching some of those IP's to see if other users are hitting those servers. Then I'd try to get the content of these emails to see if they were spam, you can impersonate your user's and check their Sent Items, if you don't have message archiving. I don't have too much more experience on dealing with a blacklisted server but others I'm sure could chime in with more info.

Notepad++ also does have a useful, search all in file and search all files. Which is very helpful when you have multiple logs open.