One-time errors:
If an error occurs only once for an address (rather than every time a post is sent), especially if the address is nonsensical and/or has no associated full name, Listserv is probably reporting that

• someone attempted to post to your list (probably a spammer) and Listserv replied 'you are not authorized to post'
• the reply bounced
• Listserv included the address in the monitor report: it'll age off in a few days. This is annoying for the list owner, but doesn't indicate a security problem with the list.

Recurring errors:
If the error isn't for a subscriber you recently deleted, the problem is probably that the person is subscribed at a slightly (or very) different address. The first thing to do is to review your list, and scan for a similar address:

• subscription may be at a sub-domain, ie. someone@mail.someplace.net rather than someone@someplace.net
• look for person by name rather than email address
• look at all subscriptions in a given domain.
• see the List Owner's Manual http://www.lsoft.com/manuals/1.8d/owner/owner.html#4.2 for more suggestions.