Egress Port 25 (SMTP) redirecting to itself (localhost)

Question

Hi,  
I'm having a strange behavior.  
  
From my EC2 Instance, if i try to:  
telnet externalhost 25   
telnet anyotherexternalhost 25   
  
It connects back to it instance's own mail server (localhost 25).  
  
I'm using a cPanel instance at Amazon, there's nothing on iptables -L -n.  
  
Also, if I try to use any other port, it works:  
openssl s_client -starttls smtp -connect externalhost:587  
  
It connects sucefully to externalhost:587, but not at port 25.  
  
Does anybody can help me?  
  
Edited by: Ramiro on Aug 18, 2021 10:31 PM  
Also, theres another thing.  
  
With root:   
I cannot telnet anyhost 25  
  
With centos (user):  
I can telnet anyhost 25, but it redirects the connection to my localhost mail server.

Accepted Answer

I'm kind of surprised that you see it going to itself, but AWS does have lots of restrictions on port 25. Have you gone through the process to get it unblocked?  
  
https://aws.amazon.com/premiumsupport/knowledge-center/ec2-port-25-throttle/  
  
While it is possible to run your own mail server on EC2 if you go through the process, generally it's a lot easier to use SES (or a competitor's mail sending service) if all you're doing is sending emails out and don't need a full mail server.

Egress Port 25 (SMTP) redirecting to itself (localhost)

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