SMTP Field Manual

smtp;451 4.3.0 Mail server temporarily rejected message. - gsmtp

smtp;451 4.3.2 Temporary server error. Please try again later ATTR17 [outlook.com]

smtp;451 4.4.0 Message failed to be replicated: No healthy secondary server available to accept replica at this time. [outlook.com]

smtp;451 4.7.0 Temporary server error. Please try again later. PRX6 NextHop: outlook.com [outlook.com]

smtp;451 4.7.1 Service unavailable - try again later

smtp;451 example.yahoo.com Resources temporarily unavailable. Please try again later [#4.16.1].

smtp;451 4.3.2 Internal server error

smtp;451 4.7.1 Sorry, the service is currently unavailable. Please come back later.

smtp;451 4.7.1 <email@example.com>: Recipient address rejected: Greylisted for 5 minutes

smtp;451 4.7.1 Try again later

smtp;451 Open relay not allowed - https://community.mimecast.com/docs/DOC-1369#451 [#]

smtp;451 Internal resource temporarily unavailable - https://community.mimecast.com/docs/DOC-1369#451 [#]

smtp;451 Invalid Recipient - https://community.mimecast.com/docs/DOC-1369#451 [#]

smtp;451 4.7.0 Temporary server error. Please try again later. PRX4 NextHop: example.com [example.com]

smtp;451 4.7.500 Server busy. Please try again later from [x.xx.xx.x]. (AS77712344) [#.prod.protection.outlook.com]