You told us generically that the email address should be authorized to post.  Could you be more specific?  Is it because s/he is an owner? A subscriber?  Specifically, please quote the configuration sections that control posting.   Look at the person's subscription. Is "nopost" set? 

It used to be possible to have multiple subscriptions with different upper/lower case letters; is this the "case"? ;-)

Finally, consider that Listserv may have different rules for considering minor differences in email addresses significant than your other systems.  For example, Google ignores dots in a username; Listserv does not.  For example, we have a FirstClass conferencing system where people think of their username as first.last, but when it sends mail to the internet, and Listserv, it uses first_last.   As you've checked this it's ot your problem.