Sent email not reaching mail server specified by subdomain MX record

The issue you're experiencing could be due to a few factors related to your MX record configuration and email routing. Here are some potential reasons and things to consider:

1. MX Record Priority: Your MX records show different preference values (10, 20, 30). The lower number has higher priority. In your case, the Amazon SES endpoint (inbound-smtp.us-east-1.amazonaws.com) has the highest priority (10). This means it should be tried first for email delivery.

2. Subdomain Specificity: You've set up the Amazon SES MX record specifically for the "m" subdomain, while the Fastmail records are for the root domain. When sending to test@m.mydomain.com, the "m" subdomain record should be used exclusively.

3. DNS Propagation: If you've recently made changes to your DNS records, it might take some time (up to 48 hours in some cases) for the changes to propagate fully across all DNS servers.

4. Email Server Configuration: Ensure that Amazon SES is properly set up to receive emails for your domain. This includes verifying your domain with Amazon SES and configuring the necessary receipt rules.

5. Sender Policy Framework (SPF) and DMARC: While not directly related to MX records, incorrect SPF or DMARC configurations can sometimes cause email delivery issues.

To troubleshoot:

1. Double-check your Amazon SES configuration to ensure it's set up correctly for receiving emails.
2. Verify that you've published the correct MX record for Amazon SES email receiving in your domain's DNS configuration.
3. Test sending emails to the full address (test@m.mydomain.com) from different email providers to see if the issue is consistent.
4. Contact your DNS provider to ensure there are no caching issues or conflicts with your MX record setup.

If the problem persists after checking these points, you may need to contact Amazon SES support for further assistance, as there might be an issue with their email receiving service for your specific setup.
Sources
Publishing an MX record for Amazon SES email receiving - Amazon Simple Email Service
Publishing an MX record for Amazon SES email receiving - Amazon Simple Email Service
Setting up Amazon SES email receiving - Amazon Simple Email Service