The Risks of Third-Party Email Clients

There are a lot of neat third-party email applications available for Mac and iOS. From an end user perspective, many of them are amazing and useful. From an information security, privacy, or legal perspective, many are horrible.

For example, Readdle makes a popular email client, Spark. Now, to be clear, I think Readdle is a good, competent, well-meaning company and that Spark is a nice app. My problem with their product isn't because I don't trust them, but because I have to trust them, and unnecessarily.

Here's why.

How first-party email apps work

When I refer to a first-party mail client, I mean Apple's own, or the app that an email service company made to support their own system (such as Google's Gmail app). These are a direct link between your computer and your email service, and are widely regarded as trustworthy and safe to use. That is, if you don't trust with your email, you probably wouldn't be using a Mac or iPhone in the first place. If you don't trust the Gmail app, you shouldn't trust the Gmail service either. A third-party app, then, is one made by someone other than the company who made your computer's operating system or your email service.

With that out of the way, here's how the process of receiving an email works on these clients:

  • A friend sends you email.
  • periodically checks your email account to see if you have new mail, then fetches it.
  • gives you some sort of notification that you have a new message.


  • A friend sends you email.
  • The Gmail mail server sends a "push notification" to your phone, waking it up and alerting it that you have new email.
  • The Gmail app on your phone fetches it.
  • The Gmail app on your phone notifies you that you have a new message.

That's straightforward.

How some third-party email apps work

Spark could have been written to work like, but Readdle chose not to, for a good reason that I understand and appreciate. All that "do I have new email?" checking can eat up a phone's battery, and if someone sends you an email right this moment, it may take several minutes before you get a notification. However, this is where a giant privacy and security issue pops up. Spark works like this:

  • The Spark app on your phone sends your email username and password to Readdle's server where it's stored until you ask Readdle to delete it.
  • A friend sends you email.
  • Readdle's server continually checks your account for new email, and then fetches it.
  • Depending on the contents of the email, Readdle's server may do some extra processing on your behalf, and may send the Spark app on your phone a push notification to tell it you have new mail.
  • The Spark app on your phone fetches your email from your mail server.
  • The Spark app on your phone notifies you that you have a new messages.

See the problem? Readdle has your login information and uses it to check email on your behalf. From their privacy policy:


OAuth login or mail server credentials: Spark requires your credentials to log into your mail system in order to receive, search, compose and send email messages and other communication. Without such access, our Product won’t be able to provide you with the necessary communication experience. In order for you to take full advantage of additional App and Service features, such as “send later”, “sync between devices” and where allowed by Apple – “push notifications” we use Spark Services. Without using these services, none of the features mentioned above will function.

By its design, you have to trust Readdle to read all your email if you want to use the Spark app, and that's not OK. Depending on what line of work you're in, it may not even be legal for you to allow another company to access your email if you don't have a signed data use agreement (DUA) or HIPAA Business Associate Agreement (BAA) in place with that company. Google will sign a BAA if you ask them. Apple's design doesn't require that because Apple never has access to your email account (unless you use iCloud email, which you shouldn't be doing anyway if you're working with HIPAA data). In fact, Apple can't access your email usernames and passwords. From their iCloud security overview:

These features and their data are transmitted and stored in iCloud using end-to-end encryption:

  • iCloud Keychain (includes all of your saved accounts and passwords)

And all of this to support push notifications, which are nice but that never had in the first place. Note: Readdle's service isn't "push" behind the curtain, as their server has to regularly poll your email service to see if you have new mail. The difference is that it's their server doing the polling using their electricity, not your iPhone. That's a handy feature, but is it worth it? In my opinion, it isn't. Further, I disagree with Readdle's statement that the "send later" and "sync between devices" features require this arrangement. They could have been built to use an end-to-end encrypted service like iCloud, but Readdle chose not to. Again, they probably did that for decent reasons because Readdle is a good company, but they didn't have to.


I'm using Readdle's Spark as an example, but mail clients are all over the place privacy-wise.

Airmail's privacy policy says:

If “Real-Time Mailbox Monitoring” is enabled for Gmail or Outlook, Office365, IMAP, and Exchange accounts, we store credentials solely to send push notifications.

Superhuman also stores your login information:

Authentication Tokens. When you sign in to the Service, we collect and store encrypted Gmail authentication tokens.

Postbox doesn't collect your credentials:

We only communicate with Google's email servers through IMAP, POP, and SMTP protocols, and never receive or store any messages or data from your Google email accounts on our servers. You can revoke Postbox's access to Google services at any time.

That's one of the less creepy terms in their privacy policy, though:

We may use information about your publicly available social media information, or your contacts' publicly available social media information, in connection with our Services.

MailMate has a clear policy:

Passwords are most often required for MailMate to access the emails in your IMAP accounts and to send emails using SMTP servers. Regular passwords are stored (if you allow it) in the Keychain of macOS. Depending on your settings, this might be an iCloud-based keychain synchronized to your other devices.

Some accounts support OAuth2 authentication. In this case, a browser is used for authenticating your accounts and MailMate only gains access to so-called OAuth2 tokens. The tokens are used to access your accounts and MailMate never sees and never stores your password. The tokens are stored in your Keychain as described above.

If an app doesn't have a privacy policy, don't use it. If it does, read the policy. And if you work in a regulated industry like finance or healthcare, get your company's legal team's opinion before using a third-party app!

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