Hi!
Please forgive me, but i don't work with Exchange logs, but from my understanding of your message, Sender-Address gives you name@domain.tld. You want to get "domain.tld" from this field in the log file. Is this correct?
If this is correct, then you should look into the EXTRACT_SUFFIX(...) function. If I remember the syntax correctly, your query would be:
SELECT TOP 10 EXTRACT_SUFFIX([Sender-Address],0,'@'), count(*) as .... GROUP BY EXTRACT_SUFFIX([Sender-Address],0,'@') ...