Emails to Comcast recipients blacklisted

@CCAntiSpam

My businesses outgoing emails - in particular from Quickbooks - are not getting to Comcast recipients. I get no bounceback message on my end; not in their inbox, not in their SPAM. Some emails have additional informational PDF attachments; some have just the one Quickbooks PDF document. It would appear that I am on some sort of blacklist. I have 2 domains - both are Google Apps domains; same issue with both. (Would prefer to private message the domain names.) 

I did a series of tests: All Microsoft domains are SPAMming me, until marked as NOT JUNK. Yahoo, Zozo, and others = right to Inbox. Checked my domains on the blacklist search site - not listed. Did verification procedure from Google Admin panel for both accounts (& waited several days / restested); no help.  THANKS.