What causes this error: Update reverse DNS failed?

The error "Update reverse DNS failed" with the reason that the IP does not exist in the specified domain can occur due to a few reasons:

1. Forward DNS record issue: Before creating a reverse DNS record for an Elastic IP, you must have a corresponding forward DNS record (A record) that points to your Elastic IP address. Make sure this A record exists and is correctly configured in your DNS zone.

2. Propagation delay: DNS changes can take time to propagate globally. If you've recently updated the forward DNS record, you may need to wait for the changes to fully propagate before setting up the reverse DNS.

3. Incorrect domain or IP association: Ensure that the domain name you're trying to set for the reverse DNS matches exactly with the forward DNS record, and that you're updating the correct Elastic IP address.

4. Permission or account lock issues: If you've previously set up reverse DNS through AWS Support, the Elastic IP might be locked to your account. In this case, you may need to contact AWS Support to remove the existing reverse DNS before you can update it.

Given the nslookup output you provided, it appears that the forward DNS (A record) is correctly set up for mx1.q***.com pointing to your Elastic IP. Here are some suggestions to resolve the issue:

1. Double-check that you're trying to set the reverse DNS for the correct Elastic IP (18...180 in this case).

2. Verify that you have the necessary permissions in your AWS account to modify Elastic IP attributes.

3. If you've recently updated the forward DNS record, wait for a few hours to allow for full propagation before attempting to set the reverse DNS again.

4. If the issue persists, you may need to contact AWS Support for further assistance, especially if the Elastic IP was previously configured with reverse DNS through their help.

Remember, each Elastic IP address requires its own reverse DNS record, so if you have multiple IPs routing to your instance, you'll need to set up reverse DNS for each one individually.
Sources
Create a reverse DNS record for email on Amazon EC2 - Amazon Elastic Compute Cloud
Reverse DNS lookup setup for multiple IPs | AWS re:Post
Elastic IP is not publicly accessible. | AWS re:Post