Hello! i have a big problem.... I am doing business over seas to denmark and suddenly my gmail address can not reply anymore to this @mac.com one. I keep receiving them, and have tried from many email addresses to reply to this gentlemen. everytime i sent it out it comes back " firstname.lastname@example.org 2:15 PM (21 hours ago) to me The original message was received at 2013-02-18 23:12:23 +0100 from postoffice.(null) [10.0.0.1] ----- The following addresses had permanent fatal errors ----- Final-Recipient: RFC822; email@example.com Action: failed Status: 5.1.1 Remote-MTA: DNS; postoffice.(null) Diagnostic-Code: SMTP;550 5.1.1 unknown or illegal alias: Thisguysemail@mac.com Last-Attempt-Date: 2013-02-18 23:12:23 +0100 i tried many times, and have reset all spam filters on my email adress and the others i tried to respond form can someone help? why suddenly after sending and receiving many emails, it dose not work?