The most frequent request.
ESTABLISH FIRST
Sending, receiving, or both? All recipients or specific ones? One device or all devices? Does webmail work?
THAT LAST QUESTION
The most useful. It separates server problems from client problems immediately.
IF WEBMAIL WORKS
The problem is their client configuration.
Check: username is the full address, correct ports, encryption, outgoing authentication enabled.
IF WEBMAIL DOES NOT WORK
Check the password, the account exists, and the mailbox is not full.
FOR SENDING FAILURES
Outgoing authentication, port, sending limits, or a rejection by the recipient.
Ask for the exact error text.
FOR MAIL GOING TO SPAM
Check SPF, DKIM and DMARC in Email Deliverability.
Check blacklist status.
Check whether the sending source is authenticated.
FOR MISSING MAIL
Check webmail, including all folders. Check filters and forwarders.
A rule the client did not create indicates compromise.
WHAT TO ASK FOR
The exact error message, a sample recipient, and approximate time.
WHEN TO ESCALATE
Server-level delivery problems or blacklisting.
