Replying to email sent to alias exposes the recipient email address

[rxgh99]

Bug report

Describe your environment

• OS name and version:

  macOS Sequoia + Brave

Describe the problem

This occurs in two scenarios:
(1) The first reply to an email sent directly to an alias address
(2) The second reply to an email “sent as” from an alias address

The target/recipient email is revealed in the “To:” field in the quoted original mail, when replying. (In Fastmail it seems to be mapped to X- Delivered-to.)

I have tested in three email services: it happens when replying to emails sent to Fastmail and Outlook, but not Protonmail.

Steps to reproduce:

1. Send an email to an alias address that maps to eg. Fastmail.
2. Reply to that email and check the quoted original mail
3. On receiving the reply, see the To: field in the original quoted email.

Observed Results:

A recipient email address is revealed.

Expected Results:

No recipient addresses revealed throughout lifecycle of email threads.