Mail that is not there.
WHAT TO CHECK FIRST
Webmail. That shows what is actually on the server.
IF IT IS THERE
A client problem: synchronisation, folder configuration, or a local rule.
IF IT IS NOT THERE
It was deleted, moved, filtered, or never arrived.
THE POP CAUSE
A device configured as POP downloaded it and removed it from the server.
Check every device's account type.
That is the most common cause of mail apparently vanishing.
THE FILTER CAUSE
Check cPanel filters and any client rules.
THE COMPROMISE CAUSE
A rule you did not create, moving or deleting mail.
Change the password immediately and check what was sent.
WHERE ELSE TO LOOK
Spam and Trash folders Archived mail Other folders, via search
FOR SENT MAIL MISSING ON OTHER DEVICES
Folder mapping. Map Sent, Drafts and Trash to the server folders on every device.
WHAT TO ASK US
If mail genuinely never arrived, open a ticket with sender, recipient and approximate time.