While there are great benefits to email’s open and decentralized nature, this freedom also makes email easy to abuse. For example, any server connected to the internet can send an email to your friend pretending to be from you. Read on to see how we can protect your custom domain from these kinds of attacks.

SPF (strongly recommended)

The Sender Policy Framework (SPF) record basically tells the world what hosts or ip’s are allowed to send email for your domain. When email servers receive email that claims to be from your domain, they can look up your SPF record and if the sending server is included. While not required, we strongly recommend you set up a SPF record that includes ProtonMail. This will not only make your email seem more legitimate and thus less likely to be sent to spam folders, but it will also help protect your domain from attackers who send emails with forged headers pretending to be you.

The recommended SPF record to add is:

The “include:_spf.protonmail.ch” means you allow the servers of ProtonMail to send on behalf of your domain. If you want to keep an existing SPF record, simply add the “include:_spf.protonmail.ch” to it right after the “v=spf1”. The “mx” also includes your domain’s MX records. The “~all” means any other servers not included should be treated as a softfail, which means accept the email but marks it as SPF failed. This is better than “-all”, which would reject emails that failed SPF, and cause delivery problems for certain legitimate emails. For example, SPF often fail during email forwarding where you send to address A, which automatically forwards to address B.Once we detect your domain’s SPF record includes ProtonMail, the SPF button in Settings->Domains will turn green.

DKIM (recommended) 

Domain Keys Identified Mail (DKIM) is a method of email authentication that cryptographically verifies if an email is sent by trusted servers and untampered. Basically, when a server sends an email for your domain, it will calculate an encrypted hash of the email contents using a private key (that only trusted servers know) and add it to the email headers as a DKIM signature. The receiving server will verify the email contents by looking up the corresponding public key in your domain’s DNS records, decrypting the encrypted hash, calculating a new hash based on the email contents it received, and see if the decrypted hash matches the new hash. If there is a match, then the email must not have changed and so DKIM passes. Otherwise, DKIM fails and the email is treated with suspicion.
It all sounds complicated but implementing DKIM for your domain is extremely easy in ProtonMail. Once you have successfully verified your custom domain, ProtonMail will generate a DKIM key pair and show you the TXT record to add if you want to enable DKIM signing. This record contains the public key and is different for every domain.

An example of a DKIM TXT record:

Notice that unlike previous DNS records, the Host / Name is no longer blank or @. It is important you add the value exactly as shown in your setup wizard and once we detect this record in your DNS, the DKIM button in Settings->Domains will turn green. We will then automatically notify you and start signing outgoing emails from your custom domain with DKIM just like we do for other ProtonMail addresses.
If you ever want to stop DKIM signing for your custom domain, simply change the DKIM TXT record you added to off.

Once we detect this change, we will automatically notify you and turn off DKIM signing for your custom domain.

DMARC (optional) 

As you were learning about SPF and DKIM, you may have wondered what exactly should the receiving server do if it gets an email which failed the checks. This is where Domain-based Message Authentication, Reporting and Conformance (DMARC) comes in to allow the domain owner to specify what should happen with failed emails as well as get feedback. Basically, there are three actions for receiving servers to take if BOTH SPF and DKIM checks fail: none, quarantine, and reject.

A basic DMARC TXT record:

The “p=” specifies the action to take for emails that fail DMARC and here, “none” basically means don’t do anything, accept the email as usual. The “rua=” is an optional parameter that specifies an email address where other email services can send aggregate reports to so you can see how many of your emails are failing DMARC.Once you are confident your legitimate emails are passing DMARC (either SPF passes or DKIM passes), then you may want to set “p=quarantine”, which tells the receiving server to send failed emails to the spam folder. Even more aggressively, you can set “p=reject” to tell the receiving server to not accept failed emails.We recommend working towards “p=quarantine” or even “p=reject” if you think you are likely to be a target of spoofing. For example, Yahoo, PayPal, and eBay uses “reject” to prevent spammers from impersonating them.

However, keep in mind there are risks for choosing these actions. For instance, when you email a mailing list which then forwards to individual recipients, this will break SPF. Annoyingly, some mailing lists also change the contents of the email, which breaks DKIM, causing DMARC to fail and thus delivery issues if you have quarantine or reject.

Test Email           

Once you have custom domain set up, you can test sending email with https://www.mail-tester.com. Send an email using your custom domain address to the unique address on mail-tester.com and check your spammy-ness score. If you have both SPF and DKIM set properly, you should be able to score 10/10!