I had a brief occurrence a couple of years ago when mail to one clump of addresses from a free hotmail account was being bounced, and it appeared to be due to the recipients' service rejecting anything with multiple recipients as "junk mail" - especially from hotmail. (Even though all the recipients were as BCC so each got only their own copy showing only their own address.)

Current advice is to be very cautious about notices of undelivered mail, especially if you get a "clump" of them as it's a common "phishing" scam. Fake rejection notices are used to "probe" your address, sometimes just to verify the address but often with a "link" or "embedded scripts" to induce you to connect to their malware distribution site. If one of your aol correspondents has been on a "mass mailing" list with a lot of CC addresses, including yours, someone may have the whole mailing list and could be "phishing it."

A quick check shows only three or four aol.com users in our contacts (of about 400 in recent off-line backups of the list), and none are recently active so we wouldn't have seen anything aol-specific.

If the addressees were included in "multiple addressee" mailings, aol may have bounced them as "junk." If this was the case, you might try sending a few of them individually addressed messages for confirmation that this was the case.

In a case of multiple non-deliveries, confirming with at least a few that they actually didn't receive your mail - using a method other than email, perhaps - might be in order if you have a way to do it, to assure that you're not getting phony notices.

I haven't heard of recent "mass migrations" of people leaving aol, although there have been a few in the past; but verifying that the addressees haven't changed email addresses (and services) is an obvious thing to do.

John