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Error: '421 4.3.2 Service not available' in the Message Delivery Logs when an email is not being delivered

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Answer

PROBLEM

The Message Delivery Logs show that an email message is not being passed on to the recipient server due to the following error:

421 4.3.2 Service not available

SOLUTION

This error can often occur when there is an issue with the receive connector settings or the Exchange transport service. Common fixes for this are:
  • Restart the Exchange Transport Service
  • Ensure that all of IP addresses are entered into the receive connector settings
IP ranges are:
  • 5.10.67.0 / 255.255.255.0
  • 94.186.192.0 / 255.255.255.0
  • 174.36.154.0 / 255.255.255.0
  • 192.69.16.0 / 255.255.255.0
  • 192.69.17.0 / 255.255.255.0
  • 192.69.18.0 / 255.255.255.0
  • 192.69.19.0 / 255.255.255.0
  • 208.43.37.0 / 255.255.255.0
  • 208.70.88.0 / 255.255.255.0
  • 208.70.89.0 / 255.255.255.0
  • 208.70.90.0 / 255.255.255.0
  • 208.70.91.0 / 255.255.255.0

CAUSE

The filtering servers have been unable to open a connection to the recipient server. 
 

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