To get an error message like that, you must be using either Windows Outlook, or Outlook Express. Those programs store your email, viruses and all, right to your hard drive. That said, as long as I can remember POP 3 has always been problematic, what with port errors and all. Try setting up your email, as strictly web mail, where you read it on the vendor's server. If something is that important, print it, or copy and paste it into a 'Word Document' and save it to your computer or removable media.

Just for what it is worth, it looks like there is some major transformation involving email in general these days. Hotmail went through it recently, where some people were seeing an outage for a month. I was one of those, as I was using 98SE and an odd Opera 10.63 browser. They seem to have that fixed now, but everything is super slow to open, with something very complicated going on in the background.
Yahoo wants everyone to upgrade from Classic email to 'new mail'. New mail will apparently never work again on 98SE, according to their system scan.

Everything looks to be up in the air again. Things I have recently seen, eBay will not allow sellers to post with 98SE, especially when it comes to photos.

Seems like the bare minimum for posting to eBay, is Windows 2000 O.S. with Firefox 6.0 as a browser. Trying to do much of anything with Windows Explorer 6 browser, is very problematic.

While I try to test various older systems online for viability, it sadly looks like anything older than Windows 2000 is no longer viable.

In order to use up an older flatbed scanner, camera and printer, I could not go up to Windows XP as drivers were not available past Windows 2000.

Bloat forced by the industry is killing it all.