Sending email reliably involves ensuring that messages are delivered without being rejected or blacklisted by spam filters, MTAs or MDAs. In order to send email reliably from EC2, there are several stategies that are available, including:
When developing an email policy with your deployments, note that Elastic IPs are considered dynamic and not static in the global registry. This can create a challenge, as some destinations MTAs (Mail Transfer Agents) carry out a reverse DNS lookup on the SMTP client to verify the source of a receiving email. When a reverse DNS lookup is performed, emails that do not match the sender's domain can be dropped.
Although this is technically outside mail RFCs and SPF should be utilized instead, it remains a common practice for mail administrators.
To stop destination mail (SMTP) servers from blocking email that originates from EC2 instances, you can map a DNS PTR record (reverse DNS) against the Elastic IP used with your SMTP server (for example, mail.mydomain.com). AWS released this support In March 2010.
Note: To apply for RDNS to be configured for your EIP, complete Amazon's Request to Remove Email Sending Limitations form.
Additionally, we recommend checking various/commonly used Real-Time Black Hole Lists (RBLs) to ensure the EIP of your SMTP server is not blacklisted. If you are blacklisted and unable to remove your IP from the blacklist, a new EIP should be purchased and applied to the SMTP server.
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