I wrote a mini app (And it really is mini - I think the title bar still has "Form1" in it) to view all the smtp errors int the event log and export them to a text file. This can help you diagnose problems sending email to a particular domain. (Just search the text file for the domain)

The app is here: www.xprs.co.uk/permafiles/eventlogsmtp.exe I hope it can be as useful to you as it is to me.

It runs on .Net 2.0

I will have to try this as soon as an email issue happens Sounds great.