If you are trying to access the same e-mail messages from multiple devices the only way to go, in my opinion, is IMAP.
The limits on storage are imposed by your e-mail service provider. I would contact them to see if it can be upped. Either Spectrum is mighty tight with space or you must have a lot of e-mail messages that you're keeping, probably with attachments that are of significant size. You're the first person I know of that's ever come up against a storage limit on an IMAP account.
Also, if you haven't done it, make sure to empty your Spam and Trash folders. Depending on the e-mail service provider and e-mail clients you're using these may not be automatically emptied at specific time intervals and you can build up a lot of storage space for stuff you already know you don't want to store.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall