Mail not sending to mac.com user

[BADDDLUV]

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

"
postoffice@mac.com
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; thisguysemail@mac.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?

[HazyCloud]

This may seem "stupid" or "easy", but have you tried emailing that address from another device or from webmail?