Exchange 2013 SMTP error "451 4.7.0 Temporary server error. Please try again later. PRX"

This error message seems to be caused by one of two different problems, unfortunately there is no documentation on what the error is supposed to mean and what the mysterious PRX codes at the end of the message should mean. This is what we found:

A) It could be a conflict between two different Exchange receive connectors.

Try these solutions

1. If you previously defined a special POPcon receive connector in Exchange please REMOVE this connector. The latest SP's for Exchange define a number of receive connectors and usually the "Default Frontend" connector conflicts with the POPcon connector. Instead: Please update the "Default Frontend" connector like this: Exchange 2013 config.
2. Check all Exchange receive connectors and make sure no two of them conflict on the same IP under "Remote network settings (receive mail from these servers)" and have the same IP Port and address under "network adapter bindings" at the same time.

In any case you'll need a reboot of the server after changing the receive connectors.

B) or a DNS issues in the Exchange 2013 set up.

From customer feedback we found these ways to solve the problem:

1. Make sure that the TLS security is OFF (unchecked) in the security options of the receive connector "Default Frontend SERVERNAME" (or the connector that is used to receive from POPcon)
2. Disable all unused network cards.
3. Disable the TCP/IPv6 protocol and use IPv4 only
4. Check all DNS server entries in the configuration of your network cards (check all network adapters) and make sure not to reference server 127.0.0.1 as DNS server but to use the real IP address instead.
5. Add an entry for each server IP address in the hosts file (C:\Windows\System32\drivers\etc\hosts) formatted like "192.168.1.1 SERVERNAME".
6. Add an entry for each server IP address in the hosts file (C:\Windows\System32\drivers\etc\hosts) formatted like "192.168.1.1 [fqdn of your exchange]".
7. Do load the latest Exchange 2013 updates, especially CU1 if you haven't done that already.
8. Make sure all servers have synchronized clocks via NTP. No really, this IS important to Exchange 2013!
9. Manually configure the Exchange DNS options: https://exchangeserverpro.com/exchange-2013-manually-configure-dns-lookups/
10. Ensure the time on the Exchange Server is exactly the same as the time on the domain controller. If the DC "lags" then this error also frequently shows up.