email blacklisted?

How does one tell if a domain name is blacklisted?
My isp appears to be blocking my customer's email (to me).
I had them send to a Yahoo acct & it went into spam folder.
I googled "blacklist" and checked a few sites but don't see them listed.
Years ago this network (buildersnotice.com) was compromised by a spammer
but hopefully not now. iSeries is handling local network & web traffic. 
Preparing
to turn on email sending on iSeries to forward to a MS Exchange server.
So far net tech not detecting Exchange problems.
jim franz