I recall that the Samsung Email App used to have trouble sending messages larger than a few MB regardless of the server’s attachment size limits. I wonder if what you are experiencing might be connected to that. There might be some information on this in Samsung’s Community.
In any case, I would recommend following a path along these lines:
Check the email account via webmail and/or other protocols than you have already used and rule out the possibility that this might have to do with how Graph API or IMAP presents the messages.
If you confirm that the same issue is reflected in the copy of the messages the server has, then I would look into the possibility of acquiring those items directly from the endpoint or any backups of it.