As usual, I left a few things out.
Gothi[c]: Not my server, but a commercial hosting account, e.g., HostGator, DreamHost, Site5, etc. If it were my server, I'd be able to cobble together a script to accomplish the task. Sorry I was unclear.
Gwen7: No longer use such services, although I did so for several years early on. Quit because one of 'em barred me for excessive usage, and another one became one of the very forces I was seeking out. Also, not significantly easier - or faster! - provided I still had the tool I used before.
Carol Haynes: Sorry, I was unclear again. Of the 500+ accounts, ~350 are active mailboxes, the remainder being just forwarders. The active mailboxes are safety net devices, in case I must re-travel the email time-line for a given account, otherwise they'd all be forwarders and this plaint would never have seen the light of the forum. As I lose the need for email history, I delete the active mailbox(es) and just use the forwarders, but there's always some new mailbox to replace one that's been retired.
Trying to think if there's anything else I assumed and left out.
The app in question, the one I've been using, was acquired some time around 98/98SE. It may have been cobbled together by one of the IT mavens on the DEC LAN at MCI (Richardson, TX), not a public app at all. If not, it was likely something available via DEC, HP, or one of the other LAN vendors, something they used internally for maintenance, so still not a public app.
I've used this, over the years, on a dozen or so different public hosts, both Windos and *nix.
Note that it did not work on Web-based mail service - GMail, HotMail, Yahoo!Mail - accounts, just server-side - mx? - accounts. I assume that was due to permissions/security issues?
It was a two-part process: part one collected the host [server] data, then part two implemented - I believe - a script. Not certain about this part, as the front-end was a compiled executable that I ran on the PC (until Vista, anyway).
Cannot think of aught else at the moment.
I know this is an outre request. Such an app has a very tiny, very restricted niche. However, when drowning, I'm willing to grasp at straws <chortle />.