Thank you for your reply.
The recipient mailbox is exist in the mail server
Let me explain our mail setup so that you can help me out to resolve this issue
I have installed iRedMail for our mydomain.com on Ubuntu Server and it is working perfectly.
Most of the users are accessing the mails via outlook and roundcube webmail. every one can send and receive mails perfectly.
The outlook configuration as follows
pop: mail.mydomain.com 110
smtp: mail.mydomain.com 587
in our mail server we have created more than 50 users.
Now the firstname.lastname@example.org and email@example.com are in different location and they are accessing their mails via Internal mail server which is configured in Remote Office. The internal mail server is downloading mails for firstname.lastname@example.org and email@example.com from iRedMail via mail.mydomain.com using POP3 service
Now the users firstname.lastname@example.org and email@example.com are accessing their incoming mails perfectly. but when they send mail, they are sending via smtp-relay.gmail.com instead of sending via smtp: mail.mydomain.com .
when they send mail from firstname.lastname@example.org to email@example.com via smtp-relay.gmail.com, the user USER2 is not receiving mail from USER1 but the USER2 reciveing mail from different domains like gmail, yahoo, live,etc.....
The funny thing is USER 1 and USER3 can send mails to all email accounts via google relay service except same domain's email accounts.
USER2 and other email accounts can receive mail from anyone except the same domain users sending mails via smtp relay service instead of using own mail server smtp service.
Getting the following error when i check the report from Google relay service
Google tried to deliver your message, but it was rejected by the server for the recipient domain <a href="http://domain.com" target="_blank">domain.com</a> by <a href="http://mail.domain.com" target="_blank">mail.domain.com</a>. [XXX.XXX.XXX.XXX]. The error that the other server returned was: 550 5.1.0 : Sender address rejected: User unknown in virtual mailbox table
If you need more details, Please let me know
Sender address rejected: User unknown in virtual mailbox table
It says recipient address doesn't exist on your server.