When setting up the Zoho Mail Addon, it is the user's choice to configure POP3 or IMAP email integration.
POP3 configuration
Emails are 'copied' to the CRM and thus reside on Zoho's server for all time, regardless of whether you delete them in your mail box. However this means every email gets copied over and users will incur email storage for each and every email copied to the CRM.
POP3 configuration has a storage restriction as mail is physically copied to Zoho servers, storage consists of emails and attachments, folders, rules etc etc. The default storage is 5GB per user.
If this runs out the Administrator would need to upgrade to a paid plan of Zoho Mail in order to purchase storage space. By doing this you can allocate more storage to certain users or equal storage to all users as you see fit.
It is possible to do this later on, but if you have a lot of emails with a lot of attachments, email folders and filters, then it will be a matter of time until you hit the email storage limit and you should factor in a paid plan of Zoho Mail too, see pricing below.
If you do not have much email use with attachments etc then you could wait see how it goes and do this step in the future. The above Zoho Mail licenses would be in addition to the addon fee but you can allocate more storage to key users than others.
IMAP configuration
If you configure using IMAP integration there is no storage restriction as IMAP means your email server handles the storage and the mail is not 'copied' to Zoho's servers, it is just synch'd with whatever is on your email server.
IMAP configuration is rather like how a phone IMAP email account works. Delete from your inbox, it's deleted from the phone too. Delete from the phone, it's deleted from the inbox.
You may wish the CRM to act like a kind of email backup tool though, in which case POP3 would suit best. At the end of the day it depends on the users usage of email and whether you think you want POP3 or IMAP configuration.