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.

user1@mydomain.com
user2@mydomain.com
user3@mydomain.com
user4@mydomain.com
-
-
-
-
-user50@mydomain.com

Now the user1@mydomain.com and user3@mydomain.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 user1@mydomain.com and user3@mydomain.com from iRedMail via mail.mydomain.com using POP3 service

Now the users user1@mydomain.com and user3@mydomain.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 user1@mydomain.com to user2@mydomain.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

Bounced
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

couldn't find the logs in iredapd.log file.

I found the information from postfix log file

Jul 12 13:02:55 mail postfix/smtpd[3166]: NOQUEUE: reject: RCPT from mail-it0-f100.google.com[209.85.214.100]: 550 5.1.0 <prvs=1731f6e63e=Jack@mydomain.com>: Sender address rejected: User unknown in virtual mailbox table; from=<prvs=1731f6e63e=Jack@mydomain.com> to=<accounts@mydomain.com> proto=ESMTP helo=<mail-it0-f100.google.com>

Jul 12 11:53:47 mail amavis[19813]: (19813-01) Passed CLEAN {RelayedInbound}, [209.85.218.43]:46328 [209.85.218.43] <user@difdomain.com> -> <accounts@mydomain.com>, Queue-ID: 18119FADAF, Message-ID: <CAA5eGRs+d3wedteg82+e_RgSf409SxYJti0+N1MEhJ+w@mail.gmail.com>, mail_id: imnKFNj6qj0s, Hits: -0.101, size: 119319, queued_as: 0D532FD674, dkim_sd=google:difdomain.com, 757 ms, Tests: [DKIM_SIGNED=0.1,DKIM_VALID=-0.1,DKIM_VALID_AU=-0.1,HTML_MESSAGE=0.001,RCVD_IN_MSPIKE_H2=-0.001,SPF_PASS=-0.001]

After fresh installation with current configuration was working fine more than 1 week . I didn't do any changes and all of sudden this issue raised.

Could you guide me to resolve this issue with your observation Please.

Do i need to do changes in iRedMail configuration?