10 1123 25 17 70 48252 1515534437 1515535037 REJECT OKWhat can be done to correct this problem?
Add the instance to the security group for the SMTP server and ensure that is permitted to communicate over TCP port 25.
Disable the iptables service on the SMTP server so that the instance can properly communicate over the network.
Install an email client on the instance to ensure that it communicates correctly on TCP port 25 to the SMTP server.
Add a rule to the security group for the instance to explicitly permit TCP port 25 outbound to any address.
Explanations:
The VPC Flow Logs indicate a “REJECT” on port 25, suggesting that the security group associated with the instance is not permitting outbound connections to the SMTP server. Adding the instance to the SMTP server’s security group and allowing TCP port 25 would enable communication.
Disabling the iptables service on the SMTP server would not resolve this issue, as the problem seems to be related to security group settings or network routing, not internal firewall rules on the SMTP server.
Installing an email client on the instance would not address the network connectivity issue. The problem lies in network configuration or security group settings, not the application layer.
Adding a rule to permit TCP port 25 outbound on the instance’s security group would allow the instance to send traffic on port 25. However, the issue seems to be with the inbound security group settings of the SMTP server, not the outbound rules of the instance.
I assess that the answer is:
Add the instance to the security group for the SMTP server and ensure that is permitted to communicate over TCP port 25.