The most common reason incoming email might not be received is the email account exceeding its quota.To resolve this, you will need to increase the amount of disk space for that email account to receive mail again.This document describes how to confirm whether you properly configured the disk space quotas on your system's devices.
However, even if you set these to unlimited you still must remain within the c Panel account's overall quota which is set by us, and depends on which hosting plan you have.
If you are already logged into c Panel and you are in the Email Accounts area of c Panel, return to the c Panel Home Screen.
Click on the File Manager icon, choose "Home Directory" and click the "Go" button.
If you are trying to increase or decrease an email account's quota an error that can occur is the Invalid maildirsize error: This can happens when the maildirsize file has become corrupted.
Fortunately, resolving this issue is a fairly easy process.