A mailbox is full
Raise the quota or clean up old messages.
When a mailbox is full, new messages bounce back to the sender.
- Raise its quota: in cPanel Email Accounts, click Manage next to the mailbox and increase the storage (within your plan's disk space).
- Clean up: in webmail, delete old messages, empty Trash and Spam, and sort by size to find large attachments.
- Using POP3? Switch to IMAP, or set the app to remove messages from the server after downloading.