Hi Zack A. Rose,
Based on the Non-delivery report (NDR) you provided, it seems that the mailbox is on litigation hold and has exceeded the Recoverable Items quota. Given your situation, I suggest you can try to increase the Recoverable Items quota for mailboxes on hold and check whether it helps. The detailed steps you can refer to the following link:
If it doesn't work after you tried the above steps, to better help you, I need to collect more information for further investigation.
- How did you send your email? (from Outlook desktop client or OWA)
- Are there any large size of attachment in the email? if yes, What's it's size?
- Will this issue occur to other users in the same organization?
- Does this issue happen to all recipient or just these four recipients?
Regards?
Bryan