Thanks for the help with the decoding.  First of all, I disabled the create account ability from the get-go, since the web mail feature was why I upgraded from the free version to the plus version probably 5 years ago or so.  Second of all, maybe it's different with the pro version since that one can handle multiple domains, but with the plus version, it was my understanding (and the way it works seems to corroborate it) that the login id is the recipient's email address without the @domain.com part.  Third of all, my biggest problem was to identify the errant account in order to identify which machine had been compromised.  With this little revelation though, I'm wondering if it's just a hacking routine correctly identifying the password as being the same as the email account (a practice I'd been using up until now for departmental addresses.)  I'm hoping this is the case, since no less than 4 machines check this address, and mine (here at home) and a notebook routinely check it from outside our firewall at work.

I've already logged into the admin panel from off-site and changed the password, so there's no need to email for what you changed it to, but thank you for the thoughtfulness of changing it nevertheless.  For the time-being, I set the smtp proxy on our firewall at work to restrict inbound smtp traffic to that where the recipient contains one of our domain names (effectively nixing off-site emailing, since the inbound smpt traffic would contain a FROM address containing our domain name - something I can undo from home once I chance the departmental address passwords from here).  Then I guess the next thing to do is to keep an eye on things for the situation to re-occur and run that decoder on the resultant logon dialogue and further identify if it's a machine issue or was just a password-hacking routine (that wouldn't even have had to resort to a brute force algorithm to figure it out).

Thank you again for all your help.

OK, maybe my wording seems a little ambiguous in light of marketing lol.  Yes, the plus version can in fact handle multiple domains, but not separately.  We currently have 4 domains registered at work, where 1 is the primary (hosted) one, and the other 3 are aliased to it,  You can enter as many domains as you'd like into the configuration, but it apparently uses them to keep any email to those domains local (without requiring a DNS lookup and communicating with another email server).  I can append any of the 4 domain names to the same email addresses, and it will short-circuit them so-to-speak to the self-same email server, but I cannot create a separate batch of email addresses for different domains.

For instance, our company name is hyphenated (Perley-Halladay Associates, Inc.) so I registered both with and without the hyphen.  I think the actual one hosted as far as websites go is perley-halladay.com, but we use the perleyhalladay.com for everything, and there are 2 others (completely different but worth registering at the time) as well, all aliased to the primary hosted one.  If I send one from...say...my address @perleyhalladay.com to another address at...say...@holdthecold.com, it knows to keep it in-house and just send it to the appropriate address set up for perleyhalladay.com, but I can't create separate addresses for the 2 domains.  That, I understand, was a feature only available in the Pro version, but then again I'm running version 1.8.8.8 I think it is, and I think the plus version is up to 1.9-something.  Hey, I've had us running an in-house email server for...like...7 or 8 years now, and the only cost was to upgrade from the free version to the 49.99 plus version many years ago in order to get the web interface the plus version offered (back before he even put ASP support in for it as I recall), and the few problems I've run into along the way, Archie has been great to help me out with.  I love this software, and the fact that it's not microsoft nor main-stream has undoubtedly saved me a ton of headaches with regards to being targeted by viruses, worms, etc.

Kudos to Mr. Gogava!