workmail mailbox unavailable

The error message "550 5.1.1 Requested action not taken: mailbox unavailable" is a common SMTP error which indicates that the recipient email address does not exist on the server.

Here are some thoughts that I have on how you can troubleshoot and potentially resolve the issue:

Verify Email Account Existence: Firstly, ensure that the email address (thomas@project604research.com) indeed exists in your AWS WorkMail setup. You should log in to the AWS WorkMail console and verify that the mailbox for the user "thomas" has been created.

MX Records: You've mentioned that you've verified the MX records using MXToolbox, which is good. Ensure that the MX records are pointing to the AWS WorkMail endpoints.

DNS Propagation: What timeframe are we talking about here? DNS changes might take some time to propagate across the internet. It generally takes a few hours, but in some cases, it can take up to 72 hours. Verify if sufficient time has passed since you configured the MX records.

Check Email Routing Policies: In the AWS WorkMail console, verify your domain’s email routing policies to make sure they are correctly configured to route emails to the intended mailboxes.

SMTP Settings: Check that your SMTP settings in WorkMail are correctly configured. Verify the server endpoint which should be in the format: inbound-smtp.<region>.amazonaws.com.

Testing: Test sending emails to the affected email address from different email providers (e.g., Gmail, Yahoo, etc.) to check if it’s a widespread issue or isolated to emails from specific senders.

Logs and Reports: Check AWS CloudWatch and WorkMail event logs to potentially identify issues or error messages that might provide a clue to the issue you're experiencing.

Recreate the Mailbox: As a last resort, if you won't lose key data, its possible the mailbox was not created properly initially or got corrupted (albeit unlikely), you might consider deleting and recreating the mailbox.

AWS Support: If you have tried all the above steps and are still facing the issue, I would recommend reaching out to AWS Support for assistance.

You might consider stepping through these items one at a time to isolate the root cause and resolve the issue. Good luck!