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Some time ago I had to migrate a mail server running multiple domains, whereby these domains were to be moved one after another instead of moving everyhing at once. That meant that the reception of mail had to be paused for specific domains only during the migration of the messages, update of the MX record and so on.

For postfix I was able to implement this using

smtpd_recipient_restrictions
   = check_recipient_access hash:/etc/postfix/access

The /etc/postfix/access file referenced above needs to contain the domain for which delivery should be deferred, along with the action defer, for example:

mydomain.com DEFER

Also, don't forget to postmap /etc/postfix/access.