Fix issue with Mail server

We are facing an issue with our mail server. For a few emails, when replying we are getting a Mail saying the mail bounced from an email address that we did not even add to the TO list.
We checked the email forwarding list and that email doesn't exist anywhere in that list. It does not happen for every email just for few email.

The bounced back email looks list this
This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:


host gmail-smtp-in.l.google.com [173.194.193.27]
SMTP error from remote mail server after RCPT TO:<salesdepartmentsectiocom>:
552-5.2.2 The email account that you tried to reach is over quota and inactive.
552-5.2.2 Please direct the recipient to
552 5.2.2 https://support.google.com/mail/?p=OverQuotaPerm h128-20020a6bb786000000b006c47cca65b0si2920401iof.53 - gsmtp
Reporting-MTA: dns; se18.mailspamprotection.com

Action: failed
Final-Recipient: rfc822;
Status: 5.0.0
Remote-MTA: dns; gmail-smtp-in.l.google.com
Diagnostic-Code: smtp; 552-5.2.2 The email account that you tried to reach is over quota and inactive.
552-5.2.2 Please direct the recipient to
552 5.2.2 https://support.google.com/mail/?p=OverQuotaPerm h128-20020a6bb786000000b006c47cca65b0si2920401iof.53 - gsmtp