Carefully examine the error message and look for varients of the email address which may be subscribed to your list. You may search it manually, but it may be best to search the information by computer. The subscriber may be on your list twice, double check using the method described above. This just means that an intermediate host or the host itself will continue to try to deliver the mail you sent a while ago. Many places continue to attempt sending mail which bounces for a week.
Some sites try for two or more weeks.
If you are receiving a very, very large volume of these messages, and based on the messages you expect to continue to receive them for another week or more, and you double check that the user is no longer on your list, you may contact the postmaster at the user's domain and ask if they could delete those messages from their mail queue so you stop receiving error messages. The subscriber may have an alias inside ListProc which is still sending them mail. You will be sent a list of the aliases for your list, if the email address is still not there, contact your ListProc Manager and ask if there is a global ListProc alias which could account for your problem. You will find that if an internet provider has a transient error, the subscriber will be removed from your list. This can be very annoying at times, but for very large lists, this is an excellent configuration option to make your listowner responsibilities easier.