Document has invalid structure

I have a user on my network that receives the error “Document has invalid structure” when he sends out emails to certain distribution lists, it only happens when he sends it to a particular distribution list, any ideas as to what could be the cause

Subject: Document has invalid structure

How large are the distribution lists?

Subject: RE: Document has invalid structure

the distribution list includes hundreds of people, and that is the reason why I do not send a test to it because of the possibility of hundreds of responses

Subject: RE: Document has invalid structure

I bet the problem is with the number of entries. Ask the user to split the groups up.

Subject: Document has invalid structure

what if you send to the list, same error? does he have the group in his pers. add. book? try resaving the group.