Error: 550 5.4.300 Message expired -> 451 Temporary local problem - please try later

[lava_za]

Greetings, 

Since moving a specific domain to Office365 mails going out to a few domains always end up being returning with the above error 451 Temporary local problem.

Then after a while the I see from the Exchange Admin center, mail flow - there was a error and then later a successful delivery.

The domain is not listed anywhere as spam, and the recipients have also white-listed the domain.

Here is an example;

Reason: [{LED=451 Temporary local problem - please try later};{MSG=};{FQDN=securemail-mx1.synaq.com};{IP=196.35.198.158};{LRT=8/25/2020 8:02:56 AM}]. OutboundProxyTargetIP: 196.35.198.158. OutboundProxyTargetHostName: securemail-mx1.synaq.com

Reason: [{LED=550 5.4.300 Message expired -> 451 Temporary local problem - please try later};{MSG=};{FQDN=securemail-mx1.synaq.com};{IP=196.35.198.158};{LRT=8/26/2020 7:22:49 AM}]. OutboundProxyTargetIP: 196.35.198.158. OutboundProxyTargetHostName: securemail-mx1.synaq.com

[#2 · Accepted solution]

This looks like a case of graylisting… best work with the other side to add an exception for your domain or similar.

[#3]

@Vasil Michev Thank you kindly for the reply, your suspicion was correct as the recipient mail server filtering had a static IP bound to the sender that was the old MX records, and hence it was permanently placing the sender in a grey-list. 

Solved by requesting a removal.

regards,