[eggsampler]

I've been a happy Fastmail customer too, until I was made aware that you can impersonate other Fastmail customers by just spoofing the email address. Their servers just happily accept it. SPF and DKIM all pass with flying colours, and the only way you'd know it's happened is if you have DMARC on and happen to notice a pass in the report you don't remember sending. Well, that is if the recipient doesn't reply to the spoofed message - hope the damage wasn't already done though. It's effectively impossible for the recipient to know it's been spoofed.

The worst part is I think Fastmail is aware of it and just don't care (believe that's why they mark their emails with a green tick and text). I understand that email has never been really authenticated, but this just throws any trust I had in Fastmail out the window.

I will be evaluating other mail hosts at the end of my subscription.

[bad_user]

SPF/DKIM are designed to prevent spam and not to prevent spoofing of From. There is a difference.

SPF has nothing to do with the From header. And the DKIM signature does not have to match the sender’s domain, the signature can be that of any domain. This means that for practical purposes, anybody can send spoofed emails. That an email is signed with DKIM, that doesn’t mean much and it is meant to build a web of trust between servers, but otherwise it is useless for the users themselves.

They wrote a blog post about how SPF/DKIM work: https://fastmail.blog/2016/12/24/spf-dkim-dmarc/

If you want to let people know which emails are from you, the From address is very weak. This is because the From/To headers tell you nothing about the source and the destination of the message, according to the email standard. Read that blog post for details.

You need a proper signature via PGP or S/MIME if you want to ensure that the receiver knows the message is from you. And unfortunately this requires education and email clients with support for such signatures (most desktop clients do), but that’s email for you.

[eggsampler]

Sure, I get that. I get there is a whole weird and wacky world of email use that is considered legitimate and needs to work that way for a myriad of reasons. I don't get why their MTA cannot at least have an option to reject mail from your domains if it's not being sent using your account credentials.

The average layperson will not get that. I'm fairly sure if my mother received an email that wasn't delivered to a their spam folder saying "Hey, remember that old copy of my birth certificate you have floating around? Could you send that. Also, CC my good friend [e-mail]" that she would call me first - if I was reachable. Also is totally ignorant of digital signatures and most likely unable to verify any present anyway.

As much as I dislike Google and try to avoid their products and services at all cost, at least I have confidence this wouldn't happen with them. Not that I would go back, but it's still concerning.

[hiccuphippo]

You should just send an email impersonating their CEO to their IT telling them to fix it ;)

[brongondwana]

That will definitely work

[Riverheart]

I'm sad to hear that but it seems to be true. I understand they have legacy stuff to support but it seems negligent for a company that exclusively deals in email to ignore something like this.

https://www.fastmail.com/help/technical/senderauthentication...

[m4lvin]

This!!

I reported the same problem to posteo.de in March 2016 and still have not received a satisfactory answer, though it seems they have some counter-measures in their webmailer nowadays. The fun part was that as a "no logs" privacy-oriented provider, they were not even able to track who sent them a complaint from their own support address ¯\\_(ツ)_/¯

As a comparison: at disroot.org I found the same problem, and it took them a few hours to repair their postfix configs.

[stefanfisk]

Do you have any reference for this issue?

[willj]

This freaks me out too, and it turns out it's true: here's a mention on their Bug Bounty page [1]:

"Email spoofing bugs do not qualify. We are quite aware that users can set arbitrary From addresses on emails, that our SPF records allow arbitrary hosts to send email as our domains, and that our DMARC policy is not enforcing passes. These policy decisions are by design, and we track the actual sender in a separate header."

[1] https://www.fastmail.com/about/bugbounty.html

[u801e]

Is there anything in the email headers that shows the authenticated user? My preference would be that email providers rewrite the From: header to match the authenticated user and that the sender uses the Reply-To: header to direct replies to a destination of their choice.

[distances]

Not good enough, as many email clients show both of these fields. The whole idea why people use this is to send and receive with one address only, even if you've authenticated with another.

[forgottenpass]

Fastmail includes a header - decodable only by Fastmail - that can identify the sender account. And IP address if sent through SMTP.

[navlelo]

What is the reason for allowing this? Laziness?

[tyingq]

They would need to make some tie between sasl authentication and what FROM: headers you're allowed to use. I don't know what MTA they use, but there are MTAs that have that feature. It's controlled_envelope_senders in postfix. I assume other MTAs have similar features.

It sounds kind of lazy to me. Though I'm sure they would get lots of complaints if they turned it on...some mailing list software depends on spoofing, for example. Or web based "contact us" forms. So perhaps it's just to avoid lots of support tickets.

[bad_user]

The reason is probably that nothing can stop the successful spoofing of the From header. DKIM is a signature for authenticating a domain, however that domain does not have to match the domain in the From header.

Take a look in Gmail at a signed email and you’ll see a “Signed by” field in its header info, with a domain name as a value.

Also the SPF setting has nothing to do with the From header either.

In other words the “From” value cannot be protected, unless you sign your email with PGP or S/MIME.

[eggsampler]

I'm not certain, maybe there's a technical reason they can validate account credentials but not map credentials to addresses/aliases. Doesn't instill confidence either way.

[aepiepaey]

Just to make it clear, is this true if you use your own domain, or just if you use any of Fastmail's domains?

[Riverheart]

Seems to be just Fastmail domains but there might be some setup involved for custom domains. See their docs

https://www.fastmail.com/help/technical/senderauthentication...

[athenot]

The From header has always been spoofable. It's just most ISPs (and Google) chose to disallow it to address low-hanging fruit in the fight against spam.

But anyone can set up their own postfix/qmail/sendmail server and put anything they want as the From.

Or am I misunderstanding the issue here?

[m4lvin]

The difference is that using another fastmail account to spoof [e-mail] will make the email look much more legitimate (DKIM and SPF wise) to other servers than when it comes from your random.emailserver.domain.foo.

[mehrdadn]

...wow. Do you have any recommendations for alternatives?

[black-tea]

Use GPG if you're concerned about authenticity. It's the only way. What you describe is not a problem with Fastmail, it's a problem with basic communication without cryptography.

The problem is if any email service did this you'd start trusting the "from" field and that is wrong. Do not trust the from field. It's as simple as that.