G Suite - Group mail Forwarding rejected by Sophos Central.

Hi,

I have the Google Suite and scanning incoming and outgoing emails. Both direction mail flow is working fine but When an outside user is sending mail on Google Group ID then this mail is delivering on the Google platform and Google is putting in forwarding pipe and the same time it is getting failed/bounced with below error on the Google Gsuite:

Bounced
Google tried to deliver your message, but it was rejected by the server for the recipient domain <a href="relay-us-east-2.prod.hydra.sophos.com" target="_blank">relay-us-east-2.prod.hydra.<wbr>sophos.com</a> [18.221.253.246]. The error that the other server returned was: 550 5.7.1 Command rejected
 
Bounced
Google tried to deliver your message, but it was rejected by the server for the recipient domain <a href="relay-us-east-2.prod.hydra.sophos.com" target="_blank">relay-us-east-2.prod.hydra.<wbr>sophos.com</a> [18.221.253.246]. The error that the other server returned was: 550 5.7.1 Command rejected
 

What is happening in the background:

Outside User sending a mail to Group ID abcgroup@mydomain.com -----> Mail received and scanned on the Sophos Central------->mail delivered on the group----> mail Forwarded from group: abcgroup@mydomain.com to Individual recipients of this group as it@mydomain.com (at this point the mail getting bounced)

We noticed an error message on the Sophos central as:

Sender local part <abcgroup+bncbc5o5shs74mbbwgb6trqkgqealgwhzy@mydomain.com> could not be validated for domain <mydomain.com>  (as Mail sender).

After a long search on the internet, I found 

1). This behavior was changed from Google sometime before to fight with DMARC “P=Reject” policy from various domains/email providers.

2). As Sophos is following RFC 5322 for incoming and outgoing emails but here looking that Google is violating this RFC ( I am not gone through the complete RFC).

 Reference URL:

https://serverfault.com/questions/779730/why-dont-my-domains-messages-to-a-google-group-get-their-headers-rewritten-so

http://onlinegroups.net/blog/2014/05/01/dmarc-taking-responsibility-sending-group-email/

https://support.google.com/mail/answer/1311182?hl=en

https://www.spamresource.com/2014/04/google-groups-rewriting-from-addresses.html

https://tools.ietf.org/html/rfc5322#appendix-A.1.3

https://webapps.stackexchange.com/questions/62737/why-does-google-change-my-from-header-to-have-via-me-in-it

https://dmarc.org/wiki/FAQ#s_3

Here, Google is saying that this is normal behavior of Google's group and it will rewrite mail header with a random ID as  <abcgroup+bncbc5o5shs74mbbwgb6trqkgqealgwhzy@mydomain.com>.  And this issue belongs to the Sophos central. 

Looking expert guide to resolving the issue.