Every mailbox draws from your account's total disk space, so quotas matter.
SETTING ONE
cPanel > Email Accounts > Manage beside the mailbox > Allocated Storage Space.
WHAT SIZE TO CHOOSE
1GB suits a typical business mailbox for several years of ordinary correspondence. 2 to 5GB for someone who receives large attachments regularly. Larger only where there is a real need.
WHY NOT UNLIMITED
Unlimited does not mean infinite; it means the mailbox can consume your entire account allowance. One person keeping every message with attachments can fill a hosting plan and take the website offline with it.
Setting real quotas means a problem is contained to one mailbox rather than affecting everything.
WHAT HAPPENS AT THE LIMIT
The mailbox stops accepting new mail. Senders receive a bounce and you receive nothing, usually without noticing until someone asks why you did not reply.
MONITORING
cPanel > Email Accounts shows usage against quota for each mailbox. Check it occasionally, or set the cPanel contact address so warnings reach you.
