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 (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
Memory is a crazy woman that hoards rags and throws away food.
~ Austin O'Malley