The following error is shown while sending mail to a domain on Plesk server or while trying to send mail from mail client software:
553 sorry, that domain isn't in my list of allowed rcpthosts; no valid cert for gatewaying (#5.7.1) (in reply to RCPT TO command))
If Plesk server is set as outgoing SMTP server in mail account preferences of mail client and this error is received when trying to send a message to an external recipient, it means that the server requires authorization and that you did not authorize before sending. Make sure that SMTP authorization support is enabled in the mail client preferences. For example, in Outlook, check authorization properties in Tools > Accounts > Mail > Mail Account > Properties > Servers > Outgoing Mail Server > Settings. There should be a checkbox option that says server requires authorization. Use the full user name for authorization, i.e., "firstname.lastname@example.org" (not just "'name").
If POP3 authorization is enabled in Plesk, then POP3 authorization can be used first (simply check for new mail), and then it will be possible to send mail within the "POP3 lock time" that is set at the Server > Mail page in Plesk.
If a bounced message is received with the same error when sending mail to a domain hosted on the Plesk server, then make sure that the mail service for this domain is enabled and that the domain name is listed in the $QMAIL_ROOT_D
/control/virtualdomains and $QMAIL_ROOT_D
If not, use the
$PRODUCT_ROOT_D/admin/sbin/mchk utility to rebuild Qmail configuration.