Check your EC2 security groups for inbound rules that allow unrestricted access (i.e. 0.0.0.0/0 or ::/0) to TCP port 25 and restrict access to only those IP addresses that require it in order to implement the principle of least privilege and reduce the possibility of a breach. TCP port 25 is used by SMTP (Simple Mail Transfer Protocol) servers for electronic mail (email) transmission: https://goo.gl/bnjFP.
Allowing unrestricted SMTP access can increase opportunities for malicious activity such as hacking, spamming, Shellshock attacks and Denial-of-Service (DoS) attacks.
To determine if your EC2 security groups allow unrestricted SMTP access, perform the following:
To update your security groups inbound/ingress configuration in order to restrict SMTP access to specific entities (IP addresses, IP ranges, etc), perform the following: