Hi @IPG87 ,
Based on my searching, there are exist two setting which could cause this issue:
- You firewall blocked Exchange online/Hotmail related IP. You could disable firewall temporarily to check whether is this issue related with firewall. If this phenomenon gone, you will need check the configuration on your firewall.
- There may be configured DNS lookup on your Exchange server
If you configured the DNS lookup, you need to make sure the DNS could be resolved to Microsoft related mail server.
You could refer to:
421 4.4.2 Connection dropped due to SocketError
Server returned '400 4.4.7 message delayed' when send to office 365
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