Start with one message
Record the sending address, recipient domain, approximate time and message ID for an approved test. Determine whether the message failed at submission, was deferred in the server queue, was rejected by a recipient, or was accepted and later filtered. Do not expose message contents or credentials in a public report.
Check the Plesk mail service and its queue using the tools available for the installed platform. A full disk, DNS failure, authentication problem or remote rate limit can all create a growing queue. Read the actual SMTP response rather than treating every delay as spam filtering.
Review the domain's DNS
Confirm MX records point to the intended inbound provider and that the sending host's identity and reverse DNS are appropriate for its IP. Review the authorized sending services before modifying SPF. Verify DKIM signing and DNS records for the relevant selector, and observe DMARC results before enforcing a stricter policy.
The Plesk mail server is not necessarily the only sender: a website, newsletter service or ticketing platform may use another provider. A change that helps one sender can break another if the inventory is incomplete.
Verify the result
Send controlled tests to more than one mailbox you manage, inspect authentication results and follow the message in the server and recipient-provider logs where available. Do not disable spam protection globally to pass a single test.
Record the issue, DNS changes and propagation checks. Some authentication and reputation problems require ongoing observation rather than a single successful message.
Further reading
Read the official documentation. For help applying these checks, explore Clouvenix services.