Problems sending via SES to Gmail accounts

The issue you're experiencing with sending emails to Gmail accounts via Amazon SES (eu-west-3) could be due to several factors. Let's explore some potential causes and solutions:

1. Temporary Gmail Issues: Sometimes, Gmail may experience temporary issues that can cause delays or failures in receiving emails. This could explain why you're only having problems with Gmail and not other domains.

2. IP Reputation: Gmail has strict spam filtering policies. If the IP address you're sending from has a poor reputation, it might cause delays or rejections. Ensure that you're maintaining a good sender reputation by keeping low bounce and complaint rates.

3. Authentication: While you mentioned that SPF, DKIM, and DMARC are correctly configured, it's worth double-checking these settings. Proper authentication is crucial for deliverability, especially with Gmail.

4. Connection Issues: The error message suggests a connection problem. This could be due to network issues between Amazon SES and Gmail servers. Amazon SES uses a fleet of EC2 instances behind an Elastic Load Balancer, and sometimes connections can become invalid when instances are replaced.

5. Greylisting: Some ISPs, including Gmail, use greylisting as a spam prevention technique. This can cause temporary delays in email delivery. Amazon SES automatically retries such messages, but it could explain some of the delays you're experiencing.

6. Rate Limiting: Ensure you're not exceeding Amazon SES sending limits, as this can cause delivery issues.

To address these issues:

1. Implement robust retry logic in your application to handle temporary connection failures.

2. Consider establishing new SMTP connections after sending a fixed number of messages or after a certain amount of time to avoid using stale connections.

3. Monitor your sending reputation and ensure you're consistently using verified domains in your "From" address.

4. If the issue persists, it may be worth contacting AWS Support for further investigation, especially if you have an appropriate support plan.

5. You could also try sending through a different Amazon SES region to see if the issue is specific to eu-west-3.

Remember, email delivery can be complex, and issues can sometimes be intermittent. Continuous monitoring and maintaining best practices for email sending are key to ensuring good deliverability.
Sources
Intermittent AWS SES Email Sending Failures (Sporadic SMTP Connection Issues) | AWS re:Post
Getting "Unverified" when sending emails with AWS SES using a verified email with my company domain | AWS re:Post
Amazon SES Delivery problems - Amazon Simple Email Service