Comment by tmarice

On their Masked Email feature page https://www.fastmail.com/features/masked-email/

> Companies have no way of linking different Masked Email addresses together to track you.

I have received the Fastmail support response, and since they do not consider this a vulnerability, I'll post it here:

- You have a Masked Email

- You have set up forwarding from your Fastmail account to another email service

- The other email service rejects the mail for some reason

- The bounce message goes back to the original sender, and may include the email addresses along the chain after the Masked Email address.

I'm assuming the bounce message contains the X-Resolved-To header mentioned in the other HN thread linked above.