If you are seeing the error 451 Temporary local problem, it may be possible that the sender is blocked due to being ratelimited.
If you are attempting to send email from one server to another, but the sender is seeing the error "451 Temporary local problem" it may be rejected by the recipient due to a Ratelimit. If this is the case, the following will be seen in the "exim_rejectlog" on the recipient's server:
temporarily rejected connection in "connect" ACL: "Host is ratelimited due to multiple failure only connections (100/1h max:5)"
If this above information is being shown in the exim_rejectlog, then the recipient will need to add the sender's mailserver IP to the file "/etc/trustedmailhosts."
Alternatively, the recipient can add the sender's IP to their Exim Configuration Manager, as a "Only-verify-recipient" value.