Users can't email a mailman list

Users unable to send email to a list they've successfully emailed in the past.

Users may receive a bounced or undeliverable message.

When you see this error it means the cached info on the users client is out of date:

How do I resolve it?

The user needs to either choose the address from the address list or type the whole address in manually. Don't use the autofill drop down that shows up when you start typing in the To: line.

For deleting emails in autofill in Outlook, please see this article: https://support.microsoft.com/en-us/help/2199226

For deleting emails in autofill in Apple Mail, please see this article: https://support.apple.com/guide/mail/delete-email-addresses-mlhlp1113/mac