Email is a communication protocol not a storage system so you should keep it clean and uncluttered. 150 MB is usually enough if you take the trouble to "tidy" your emails regularly.
There are THREE ways to deal with a FULL email box:
1. Forward everything to a free service such as GMAIL for long term archiving then delete all but the most current from your regular email box. Remember to delete all the folder contents: INBOX, SENT and TRASH.
2. Keep the email messages but detach all the ATTACHMENTS.
3. Save your current emails to a LOCAL FOLDER. This means all your emails are now saved on your PC or laptop and not on the email server. You will not be able to access these emails from other devices, such as your phone, but you will always be able to access these archived emails from the local foolder on your computer. Doing this you free up your IMAP server email allowance and your computer storage space is huge by comparison.