Tell HN: Fastmail is leaking internal routing email address/username with teams
Using throwaway...

Fastmail is leaking the distribution mail address/username via the X-Resolved-to header.

Now, normally this wouldn't be such a big problem, but when someone (needs to) forward(s) the original email as attachment, the internal email addresses are revealed, which is many cases is your Fastmail username. In my opinion this is a security / data leak. Fastmail's "fix" is to manually remove the X-Resolved-to form the email source. Of course this is ludicrous because:

1) People in your team would never do that, and non-technical people wouldn't even know how to.

2) It's easy to forget.

3) It changes the email, so any signature will not be valid anymore, marking it as corrupted.

I asked to escalate the issue when support couldn't reproduce (not sure why not, as my screenshots were clear, and their KB article explains that these headers exist). Even though it has been "escalated" to the "Support Lead - customer/technical" the "solution" still is to remove the header manually.

The header is of no use for Fastmail's customers, and is probably only used to Fastmail's internal routing for the team-mailbox-feature. It actually makes me second guess if JMAP is even properly designed. I'm very disappointed by the way this has been handled, as I clearly described why this is an issue.

Over the course of two weeks:

Reply 1:

> The X-Resolved-To header is only added to incoming emails to Fastmail, hence only you will be able to see this header. It is not applied to outgoing emails, that is the X-Resolved-To header is not passed along when forwarding an email or replying to an email.

> However, I have shared your suggestion with our development team as a feature request. Please note that we're unable to offer any timeline as to if or when this feature could be implemented.

Reply 2:

> I'm afraid I was unable to reproduce the issue you are seeing. When forwarding an email as an attachment and later checking the headers of the attached email, I could not find the X-resolved-to header nor was my Fastmail username mentioned in the headers.

> Therefore, could you please share a screenshot of the issue?

> On the other hand, I understand that your Fastmail username will be exposed when sharing a screenshot of the email headers. Therefore, I recommend refraining from doing so. If needed, you could copy-paste the headers to a Word doc, remove the X-resolved-to header and then share the screenshot.

Reply 3:

> I’ve escalated your ticket to a Tier 2 agent, as they are best suited to assist with this issue. Please note that if you have a complex issue, or if they’re handling a large number of tickets, response time may be longer.

Reply 4:

> I’ve escalated your ticket to a Tier 3 agent, as they are best suited to assist with this issue. Please note that if you have a complex issue, or if they’re handling a large number of tickets, response time may be longer.

Reply 5:

> Unfortunately there is currently no way to remove the X-Resolved-to header if you forwarding the raw form of the email. You will need to manually remove the header, by downloading the email, opening it in a text editor and then attaching the email again. Note that, the email authentication is checked against the email and not the attached ones, so that shouldn't fail the sending email authentication. However, if you are doing any checksum against the original email content, then yes that would fail because of change in headers. I am sorry, but there is currently no work around to this