sendmail DSN Server unavailable a day after the AWS maintenance reboot

If you're experiencing issues with sendmail after an AWS maintenance reboot, there are several potential causes to investigate.

When an EC2 instance undergoes a system-reboot maintenance event, it's migrated to a new host. While this typically completes in minutes rather than hours, it can affect certain configurations that depend on the underlying hardware. The migration preserves your IP address, DNS name, and data, but the change in physical hardware could potentially impact services like sendmail.

For your sendmail "DSN: Service unavailable" error, I recommend checking the following:

1. Verify that your DNS settings are still correctly configured. Since you mentioned the issue appeared a day after the reboot, it's possible that DNS resolution is failing.

2. Check if your sendmail service is properly configured to start after reboot and that all dependencies are satisfied.

3. Examine your sendmail configuration files to ensure they're still valid after the reboot.

4. Review your network security groups and firewall settings to confirm that the necessary ports for mail services are open.

5. Check if there are any resource constraints on your instance that might be affecting sendmail.

Posting your nslookup results, resolv.conf, rc.conf, and dnschecker.org reports would indeed be helpful for diagnosing the issue, particularly to verify DNS resolution which is critical for mail delivery.

If these basic troubleshooting steps don't resolve the issue, and you have a paid AWS Support Plan, opening a technical support case with AWS would be advisable to get more specialized assistance.
Sources
Question about system-reboot | AWS re:Post
Amazon SES Delivery problems - Amazon Simple Email Service