Check if the sender domain exists

It's somewhat common check if a sender domain exists before accepting mail. This check is technically backed by the standard[1] however in practice it is prune to false-positives, so we can't fully endorse this method of anti-spam prevention. The standard argues that since mail should be transactionally safe, the return/sender address has to be valid so that non-delivered mail can be returned to the sender.

if ($senderdomain != "") {
    $dnsa = dns($senderdomain, ["extended_result" => true]);
    $dnsmx = dnsmx($senderdomain, ["extended_result" => true]);
    if ($dnsa["error"] == "NXDOMAIN" and $dnsmx["error"] == "NXDOMAIN") {
        Reject("$senderdomain doesn't exist");
    }
}

This check, can be done in the MAIL FROM context or at any later stage.

[1] https://tools.ietf.org/html/rfc2505#section-2.9