Internal server error when accessing email through Webmail

Keep the exact message Webmail shows. Internal Server Error is a generic error and doesn't prove a lack of space; disk quota exceeded, on the other hand, lets you start with the quotas.

If it says the quota was exceeded

1. In cPanel > Email Accounts, open Manage for the mailbox and compare its usage with the assigned quota.
2. Increase the quota only if the hosting plan still has space available. Enlarging the mailbox doesn't create extra capacity on the account.
3. Check Disk Usage to see the hosting service's total usage.

If it only shows Internal Server Error

Note down the URL, the time and the action that caused the error. Try once from a private window to rule out a broken session, but don't blame the quota if the message doesn't say so. If it persists, contact support with those details so the Webmail, PHP and mail logs can be reviewed.

Keep your messages while it is checked

If you have a draft open, save its text before signing out or clearing cookies. A Webmail fault doesn't prove the emails have disappeared: if you already have an IMAP client set up, check there whether they are still accessible.

Don't delete the mailbox's internal folders from File Manager to try to fix the error. If the log points to another limit, such as the number of files, that specific limit has to be checked even if there are megabytes left. Any clean-up must keep the messages you need.