Temporary rejection due to greylisting. This technology is only applied to new IP addresses which do not have a (good) reputation yet in our global systems. We do not apply "classical greylisting" so this should not cause any delays on your legitimate traffic.
For new Local Cloud installations please allow up to 72 hours for the systems to "learn" about your traffic.
Messages which have been temporarily rejected, stay stored on the sending mail server. Legitimate mailservers always automatically retry delivery of such messages. Depending on the reason of the temporary reject, the message could get accepted at a subsequent delivery attempt. It's always possible to whitelist the sender to disable any checks and to ensure that the message will get accepted as soon as it's retried by the sending server.