Hi, I access my inbox from a few different appliances (Laptop, Phone, PC), so IMAP is preferred for this. However my email provider has a mailbox size limit and, so, I can't just keep all my emails that I do not want to delete. Once I have read them, I do file them away in separate folders, but these are part of the mailbox and, thus, still count to the size.

In a perfect world, is there a way I can have the inbox treated as IMAP (i.e. accessible to all appliances) but the other folders in my mailbox treated as pop3 (where I would be using my laptop to take them from the mailbox to store them so I don't have to delete them)? Or be able to select individual folders to be treated as pop3?

I have had it suggested to me about trying, once a month or so changing the setting to pop3 to download everything and setting it back to IMAP, but I would need to put everything that was in the inbox back in... Rather clunky.

You can not selectively POP3 from hosted folders. You can however copy messages from a folder in the IMAP connected mailbox to a local folder (one in My mailbox).

Alternatively, consider this...

Let's say you want email stored on your laptop. You configure it to use POP3 with the option to leave mail on the server enabled. Your PC and phone can then access that mail via IMAP. Periodically you do deletions on the host to keep used space in check. Deletions can be done with one of the IMAP connections or by connecting to your host via their web interface and doing deletions there. A disadvantage to this method is that copies of messages sent from the PC will not be visible to the laptop or the phone, unless the host is Gmail. Gmail keeps a copy of messages sent via their SMTP in a folder named Sent.