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If you read https://sendersupport.olc.protection.outlook.com/pm/troubleshooting.aspx, it says

 Confirm that your DNS is set up correctly
 Try connecting to mail.hotmail.com via port 25. If you are unable to connect, then attempt to telnet over port 25 directly to our email servers (MTAs). You can find the current list of our MTAs by querying "nslookup –q=mx hotmail.com" from a command prompt (this should work in a variety of Operating Systems). Currently, the addresses for these servers are mx1.hotmail.com, mx2.hotmail.com, mx3.hotmail.com and mx4.hotmail.com. If that doesn't work, try connecting directly to the IPs. If you are able to connect directly to the IP and not mail.hotmail.com, then it is likely that there is an issue with your DNS server.

So how to connect to mail.hotmail.com via port 25 from DNS?

cilapo1541
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  • By using Telnet, as it says in the instructions you pasted here. https://docs.microsoft.com/en-us/exchange/mail-flow/test-smtp-with-telnet?view=exchserver-2019 – Rob Moir Jun 14 '20 at 08:39

1 Answers1

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Those are two separate instructions...

You need to confirm that your dns is set up and working correctly and that mail.hotmail.com resolves.

For instance nslookup mail.hotmail.com or dig mail.hotmail.com or even ping mail.hotmail.com Should return an ip-address and not result in hostname not found error or similar.

When it does: then you need to confirm that you can establish a connection to TCP port 25 (the port used by the SMTP protocol for mail delivery) to mail.hotmail.com. You can use telnet for that.

Bob
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  • nslookup and dig work and return IP. But telnet to mail.hotmail.com at port 25 returns nothing. [root@server ~]# telnet mail.hotmail.com 25 Trying 65.55.72.183... – cilapo1541 Jun 14 '20 at 16:38
  • That is likely the root cause of your problem, outgoing traffic on port 25 is blocked (by your own firewall rules, your corporate or ISP network or different). – Bob Jun 14 '20 at 20:27
  • I actually thought most servers/ISP block 25 due to security. I dont know why Microsoft wants us to test on port 25. – cilapo1541 Jun 15 '20 at 07:04
  • As I said - mailservers use the smtp protocol to deliver mail from one mail server to another - smtp uses port 25 - and when that is blocked you obviously get an error – Bob Jun 16 '20 at 06:30