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AWS Port 25 block – Why does it happen and how to fix it?

AWS Port 25 block – Why does it happen and how to fix it?

From payment confirmation to support queries, emails play a vital role in online business communication. Delayed alerts or lost orders end up adversely affecting the business.

In AWS EC2 instances, mail delivery problems can happen due to many reasons, mainly port 25 connection limits, IP blacklists and port 25 blocks.

Read: How to fix Amazon EC2 IP blacklisting in Spamhaus PBL and other email RBLs

Today we’ll see the different reasons for mail delivery failures and the ways to resolve them.

1. Port 25 throttling

Amazon sets limits on the number of emails that can be sent from a mail server, as a method of spam prevention. This throttling can lead to email delivery issues.

To remove the limits on the number of connections, one needs to contact Amazon and submit a request. Once Amazon approves the request details, they would raise the limits.

2. IP blacklisting

It has been noted that IP addresses on Amazon EC2 get occasionally blacklisted in Spamhaus and other such lists. In such cases, to ensure email delivery, immediate action required is to change the mail interface IP.

Amazon provides additional IP addresses upon requests. This extra elastic IP can be assigned to the mail server, if the existing IP is blacklisted.

Some anti-spam companies validate a mail server using its RDNS record. As a proactive measure, it is important to contact Amazon and set RDNS for your mail server.

Read: How your web hosting business can keep out of spam blacklists


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