The users keep getting a NDR 5.1.1 message from a user who has been deleted from AD over 6 months ago & is no longer a member of the particular group. The username does not show up in AD when you do a search. The NDR activates only when a meeting request is sent for this particular group.
Veeam is happy to provide a free NFR license (for 1 year, up to 10 users). This license allows for the non‑production use of Veeam Backup for Microsoft Office 365 in your home lab without any feature limitations.