I had the same thought as Charlie, Anne-Marie, based on what very little you told us. You probably used something like "owner" or "admin" in the name of the special account.
I had one member on my last list who made the short-sighted choice of "mkt_admin" for his logname, and his ISP allowed it.
Of course, my first action was to make sure that it was a personal account and not a job-title address; my experience with job-title addresses is that someone else succeeds to the position and the email address, doesn't expect your mailing list, and tries to have you strung up as a spammer, so I tell people trying to join under such addresses to reapply under their personal accounts. He swore to me up and down that it was his personal account and that the address was his, not his job's.
So after I let him on, he began to post. All his posts were taken as mail from a daemon, and after manually rescuing a few from the list's -request box, I finally set up special recipes to get his posts through. Then his address became invalid and I never heard from him again; what a waste of effort. After that my policy was simply to disallow such addresses and in- sist that people subscribe under non-special ones, but I never had another like that, so I never had to enforce it.