Server Rejected by DNSBL
Under the Warden -> Reject log you see an entry being rejected by one of the DNSBLs you set in Plesk under Tools & Settings -> Spam protection based on DNS blackhole lists -> DNS zones for DNSBL service.
You can temporarily whitelist the server by clicking on the Client Addr -> Client Access Tab -> Select OK permit the request -> Press the Update button to save it.
Important
You should still tell the sender to inform the postmaster of the blocked server to start the DNSBL removal process as their mail would still be getting blocked by other servers that use that same DNSBL. After the server is removed from the DNSBL you should remove the whitelist entry under Warden -> Settings -> Mail Server -> Mail Server Access -> Client Access.