Symptoms
You are attempting to send an email from one server to another, but the sender is seeing the error "451 Temporary local problem." 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)"
Description
If you are seeing the error "451 Temporary local problem," it is most likely that the sender is blocked due to being rate limited.
Workaround
You will need to either wait for the recipient server to release the rate limit automatically or contact them directly to request the limit be removed.
If your cPanel server is rate limiting a sender that you wish to whitelist, the recipient can add the sender's IP to the Exim Configuration Manager Basic Editor as an "Only-verify-recipient" value. Alternatively, you can manually add the sender's mail server IP to the file /etc/trustedmailhosts.