Mail issue with Groups

We have a group “Main Group” which has 7 sub group which contains users. An email was sent to this Main group but only 50% users received the mail not all of them.

We also have message tracking enabled which gives no info on users who did not get that email. I have checked Console logs but no luck.

Checked those users mail file for quota issue etc but all are fine now

Is there a way I can investigate why those 50% users did not receive that mail

Please suggest how to troubleshoot the issue

Subject: group type?

are and of the sub groups set to a group type of ACL Only? Do all members of a particular sub group get the message or is the entire sub group not getting the e-mails?

Subject: Re: Group type

I checked and confirmed that all groups are Multi-Purpose type.Helpdesk gave me 3 names of users as an example and I checked that two of them belong to one particular group but all looks OK with the group

Subject: Groups in Local Contacts

I often find with this type of issue that users who send to these groups have copied the group(s) to their local Address book and they have got out-of-date over time. Worth checking