I have been a paid customer for both (Posteo for 2 years, then moved to ProtonMail Visionary to my own domain names).
Why would someone fake mail from [free email provider] when they could just register an account?
DMARC is a new technology
A fake or subversive email from a person registering an account with a different looking email address of the person they are impersonating would be more likely to be caught as a fake than a spoofed, received email from what appears to be the real email address of that person. Spoofing the email address would be more successful in some cases. Also, DMARC is far from new and dates essentially over a decade old.
While not demonstrated anywhere on the Posteo FAQ about DMARC, I would personally imagine they don't care that much because Posteo constantly remind you to use PGP for emails between others, which can be used to 'sign' a message to verify that this email was written by the legitimate owner of that email address, providing you had their public key before they sent an email to you.
Posteo's mailbox encryption means they cannot see the emails and metadata, however because of how it is implemented (being able to use a custom E-Mail client) the mailbox has the possibility to be decrypted during an authentication, while they don't know the keys to decrypt your mailbox, this wouldn't really matter since:
- The service posteo provides isn't secured by zero-knowledge/zero-access methodologies.
- Posteo isn't transmitting the encrypted mailbox to you, why do you think using any email client works? No email client app just supports recieving Posteo's unique crypto-mail emails to decrypt on your local machine. The only encryption protecting viewing the emails and their contents is the TLS encryption between you and Posteo while you are authenticated. If the TLS connection was monitored by Posteo via interception they could also see it.
Posteo also claim this themselves:
With crypto mail storage enabled, your emails will only be decrypted for you the moment they are accessed.
Because the emails are first encrypted when they reach our servers, Posteo crypto mail storage is no substitute for regular end-to-end encryption set up by the sender of an email. This does not, therefore, protect you from a lawful interception (TKÜ).
ProtonMail's service when it comes to mailbox storage is zero-access and they cannot read contents of your mailbox at all and the usage of a ProtonMail app is required to assist that. They imply this in Transparency also, while in Posteo's transparency they have disclosed mailboxes, mainly due to Crypto Mail Storage not being turned on, or by an interception occuring.
The only flaw ProtonMail really does have, like mentioned with Posteo, is the lack of end-to-end encryption used for email transit, where instead it is encrypted with TLS using certificates maintained by the service. This would only truly apply to emails that are not PGP encrypted or other ProtonMail email addresses. However, this is a flaw with email as a whole (no end-to-end encryption), not a flaw of each individual service, so it would be wrong to criticise both on these fronts.
I would suggest ProtonMail above everything but if spoofing isn't part of your threat model I'd say it's sufficient... but considering how Proton provides just as well of a service for free, it would be strange in my opinion. I would like to see Posteo add a DMARC policy soon though, maybe I'd be more favourable if it did.