Hope I can ask this clearly. I have 5 email accounts in play. A@A.com is a new one set up with ISPConfig3 on a server in a data center. B@B.com is an existing one with a shared hosting provider. C@C.com is an existing one set up with ISPConfig3 on a home server and relaying though B@B.com's mail server. D@D.com is a webmail account with Yahoo. E@E.com is a Microsoft Exchange Server at the day job. A@A.com is the issue. It can receive emails from all of the others. But if it sends an email to the other 4, it arrives fine at D@D.com and E@E.com but never arrives at B@B.com or C@C.com. The postfix log says all passed clean with no errors. There is never a return message saying that there was a problem anywhere. They just vanish. I'm thinking the shared hosting provider is somehow blocking these but now sure how or why. The IP and host name for A@A.com don't appear on any blacklists. Anyone have a clue of what else to check out? Thanks.