By default Mail::Sender stops trying to send the message as soon as the server reports any problem with any address, whether it's in To:, Cc: or BCc:, in this case your mail server started to report even problems it did not before. You either have to handle such errors, remove the invalid address and try to resend the message. Mail::Sender DOES give you enough info to do that within the script.

Or you can use the skip_bad_recipients option and then review $sender->{'skipped_recipients'} to get a list of addresses that failed. With that option set to true, Mail::Sender only fails the message and reports an error if ALL recipient addresses fail.

HTH, Jenda