Virtual1, thank you for your reply.

It's only me on both machines, no-one else uses them. I only use the old Mac and Outlook Express under specific situations which only arise once a week for our business: the rest of the week the iMac and Mail does the day-to-day emailing traffic.

Tacit, thank you! You got me thinking, so I just went over to the G4 and rummaged around Outlook Express. Sure enough, when I finally found it, under my account I had NOT ticked the box and all downloads were immediately deleted off the server. That's where the pesky little blighter was. Thank you.

I've now re-set that, so no emails are deleted off the server automatically.

End of conundrum. Case solved. Another Hi-Five for MacFixit Fine Tuned Mac.