System Administrator Bounce Message

I sent an email to 12 of my contacts; all on BCC line and received the following message from System Administrator :

Your message did not reach some or all of the intended recipients.

      Subject:     Some old some new

      Sent:  4/25/2026 1:10 PM

The following recipient(s) cannot be reached:

My issue is that no recipients were listed so I have no idea who did or did not receive the email or why.  This type of System Administrator email notification generally tells you why the message was undeliverable; permanent failure, mailbox full, over size limit, etc. but none of that was mentioned.