Pending messages when email sent in to shared group

 Hi All, 

Our organisations runs an on premise version of exchange 2013 CU 21 with 12 servers and 4 DAGs (3 servers per DAG)

A few of our servers were recently migrated by a third party over to a new site/data center and ever since the migration we have encountered numerous issues.

The issue i am looking in to at the moment and hoping to get some help on is that we have a universal mail enabled global IT security group with 3 mail enabled global security groups as similar to the below

(G) BHV (GL) IT Infrastructure  (Universal Security)

Inf UK (GL Security)

Inf Asia (GL Security)

Inf US (GL Security)

When a member of the team, not just 1 member but all sends an email in to the group, sometimes it will send fine to all 31 recipients, but when sent again later it will only send to 18 or another amount less than 31. We have viewed this via running the delivery reports from ECP, the messages that don't send have a state of pending.

I have Microsoft on the case also, but dont seem to be getting very far at the moment. 

Thanks