Start by separating a login problem from a delivery problem. A message in your Sent folder shows that the app saved a copy; it does not prove the recipient received it. Keep the full delivery failure notice when one is available.
- Identify the direction of failure: incoming, outgoing, or both. Note whether it affects one mailbox, the whole domain, or only a particular recipient provider.
- Open the website's Email page in Control Center. Check that the mailbox is Enabled, then test Open webmail with its displayed Mailbox username. If webmail works, review the app's hostname, password, port, and security mode. Check outgoing SMTP separately.
- For missing incoming mail, check storage, all folders, and forwarding/filtering rules. Under Connect mail DNS, select Check mail DNS and review missing or different records. Confirm that the authoritative MX records point to the intended email provider; preserve another provider's records if mail stays there.
- For outgoing failures, read the returned diagnostic text. An unknown-recipient response suggests checking the destination address; a quota response suggests storage; an authentication or policy response needs the sending configuration investigated. Temporary and permanent failures require different handling, so keep the exact code rather than paraphrasing it.
- Check SPF, DKIM, and DMARC for mail that reaches Junk or is rejected for authentication. Sender reputation and message content can also affect delivery. Follow the receiving provider's guidance when the bounce links to it.
- Send one small plain-text test without attachments. Use a separate external mailbox, note the timestamp and timezone, and ask the recipient to check Junk. Avoid repeatedly resending sensitive or large messages.
- If unresolved, contact support with sender and recipient addresses, timestamp, message ID if available, error text, and whether the problem occurs in webmail. Share headers privately and omit passwords.
Check the result: Confirm the recipient received the test and can reply. A matching mail-DNS report or an issued website SSL certificate is not a delivery test. Test the original sending route after a correction; a mailbox test does not verify a website form's configuration.
Common problems: Recent nameserver changes can omit mail records. Expired domains can interrupt routing. A full hosting account can affect mail even when one mailbox's apparent quota looks sufficient. Support can inspect server logs, queues, and reverse DNS; customers should not change server-wide mail services. Virtualmin email troubleshooting
Related: Email app setup, Check mail DNS, Restore mailbox sign-in, Authentication records.
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