Email generates more reseller support tickets than everything else combined. Most are the same handful of issues.
CANNOT SEND
Outgoing authentication is not enabled in the mail client. It must be. The network blocks port 25. Use 465 with SSL or 587 with TLS. The username entered was the part before the @ rather than the full address.
CANNOT RECEIVE
Mailbox is full. Check the quota in cPanel. The account's disk quota is exhausted, which stops mail delivery regardless of the mailbox quota. MX records point elsewhere, or email routing is set to remote when it should be local. The message is in the spam folder.
MAIL GOING TO SPAM
Check SPF and DKIM under cPanel > Email Deliverability and repair them. Check the site is sending through authenticated SMTP rather than PHP mail. Check the From address is on the client's own domain, not a Gmail address.
DIAGNOSING QUICKLY
cPanel > Track Delivery for that account shows whether a message left the server and what the receiving side said. It settles "it was never sent" arguments in seconds.
Escalate to us if Track Delivery shows server-side failures you cannot account for.
