I have identified a misconfiguration in the way e-mail is set up at my workplace. We use POP3, and the accounts at our e-mail provider are set to move spam to a Junk mail folder. I have just learned that POP3 does not understand folders (only sees Inbox), so junk mail is never downloaded - all of it stays on the server until it is auto-deleted.
This explains why, for years, e-mail from customers would just not show up every now and then. Adding to the confusion, Outlook performs its own outdated non-bayesian filtering and does place things into the local spam folder, making it look like things are working as intended. I only discovered the issue after peeking into the serverside webmail interface.
So how do I remedy this situation without causing too much disruption?
I could change the serverside spam handling to 'place spam into Inbox', however I believe that would be the same as just turning serverside spam filtering off, defeating its whole purpose.
I could change the protocol to IMAP, but then I'd have to find a way to make Outlook still appear to behave like a POP3 client. Employees keep upwards of 20GB of e-mails in their primary .pst file. I have no idea how this setup could translate to IMAP without being incredibly awkward to use. Our provider gives us limited serverside storage, so keeping it all serverside is not an option.
I could theoretically set up some kind of locally hosted e-mail storage or proxy service on our server, but I don't even know if such a thing exists. I definitely want to avoid running a full blown e-mail server and all the day-to-day hassles which come with that.
None of these feel like the right solution, that's why I decided to ask here.