Confusion about group/user emails

[MAinfo]

For several of our admin users, when external accounts send email, the emails go through, but the sender receives a response that their message could not be sent because the group they tried to contact doesn't exist...permissions...etc. I need help figuring out how to resolve this. 

[newman]

Is the email being sent to a group?  It might be an issue with nested groups.  The main group allows external posting but a member group does not.

I would look at the email logs and see if that gives you any insights.   

If it was sent to a group I would check to make sure no members of the group no longer have accounts, and check the settings of any groups that are members.