408 karma · joined January 12, 2016
Just to clarify, in Fastmail, you have the option to add alias email addresses to your Fastmail account. It's like in Gmail where you can add a "+" or "." after your primary email's local part, and it will still get delivered to your original Gmail inbox. This is like that but you can assign any addresses to forward to your original Fastmail account. They even advertise this as a privacy feature[0] (though I'm not sure if they have different treatment than Aliases[1])
What OP was saying is that Fastmail internally adds an email header that reveals your original Fastmail account during a process to route your emails. This header will be presented if you then reply to an incoming email sent to an alias or masked address, thus leaking your original email to the receiver.
[0]: https://www.fastmail.help/hc/en-us/articles/4406536368911-Ma...
[1]: https://www.fastmail.help/hc/en-us/articles/360060591073-How...
`sudo xattr -d com.apple.quarantine $BINARY_NAME`