INBOUND throttling by default on port 25?

Question

I have been trying to set up an email server to RECEIVE mail on an EC2 instance for weeks without any success. I am well aware that there is throttling on port 25 for OUTBOUND connections and I understand the reasons for it. But why is it closed INBOUND and how to resolve it?

Answer

Were you able to Telnet to port 25 with a HELO test to verify if the mail service is listening? Also have you had your MX record set up so that the internet knows where to route the email to? You can use the EC2's IP to do a simple test here https://mxtoolbox.com/diagnostic.aspx.

Answer

Ensure that the inbound traffic on port 25 is allowed in the security group associated with your EC2 instance to enable receiving mail.

Answer

Things you need to receive email on an EC2 (assuming Amazon Linux, public ip and subnet for the scenario)

1. Security group open for port 25
2. smtpd listening on the instance on port 25 (postfix would be my choose)
3. smtpd configured to receive emails for your domain
4. MX record pointing to your server in Route53 (or other DNS setup)

If you want to read the mails remotely, then add a pop/imap configuration. Or just use mutt on Linux.

INBOUND throttling by default on port 25?

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