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Your mail server returns this error: 451 4.7.0 Temporary server error. Please try again later. PRX5

Two suggested possible solutions

  1. ex2013 sometimes encounters  problems when you leave the DNS lookups setting in the server properties to "All network adapters", instead you should select a specific NIC or enter the IPs of your DNS servers manually.  ECP only configures the transport service, but NOT the the frontend transport service. you can check this with get-transportservice and get-frontendtransportservice and looking at the ExternalDNS and InternalDNS settings. Usually onl theTransport Service would adopt the settings applied to the ECP. Please also configure set-frontendtransportservice with the same DNS settings as the transport service.
  2. Add the local server's IP to your hosts file: server server.domain.local  Restart "Microsoft Frontend Transport" and "Microsoft Exchange Transport" services

( [Online] Microsoft, 2015, Date viewed: 30th August 2015, Ref: https://social.technet.microsoft.com/forums/exchange/en-US/fc26dac5-d4e2-49da-903d-361ea8b85388/451-470-temporary-server-error-please-try-again-later-prx5)