Mozilla is rolling Thundermail, a Gmail, Office 365 rival

Thunderbirds are Pro: Open-source email client to get message hosting, appointment scheduling, more

Thunderbird, Firefox maker Mozilla's open-source email client, is aiming to reinvent itself as a more comprehensive communications platform.

Toward that end, the development team is in the process of rolling out a set of adjacent services, some of which may eventually bring in revenue.

The storm bird's ecosystem expansion campaign represents an effort to counter the gravitational pull of commercial services including Google's Gmail and Microsoft Office 365, which have been peeling off Thunderbird users bit by bit.

"Thunderbird loses users each day to rich ecosystems that are both clients and services, such as Gmail and Office 365," explained Ryan Sipes, managing director of product at Thunderbird, in a planning note last week for the development community.

Thunderbird loses users each day to rich ecosystems that are both clients and services, such as Gmail and Office 365

"These ecosystems have both hard vendor lock-ins (through interoperability issues with 3rd-party clients) and soft lock-ins (through convenience and integration between their clients and services). It is our goal to eventually have a similar offering so that a 100 percent open source, freedom-respecting alternative ecosystem is available for those who want it."

Those losses aren't easy to measure since Mozilla does not publish user numbers for Thunderbird. But monthly active installations have dipped from 17,706,777 on December 27, 2020 to 16,174,806 on March 30, 2025.

Sipes told The Register those monthly user stats don’t quite capture the entire user base, and there’s an effort to represent those numbers more accurately.

"We know that we have gained users in the last few years overall: We have around 20 million monthly users," he explained to us.

"For the last ten years, we have seen a large move to webmail from dedicated email clients. This is in part due to the ecosystems around those email services, not just the mail but documents and file storage and sharing, etc. We want to close that gap and create an open source ecosystem, based on open standards.”

Sipes said the Thunderbird team aims to move email into the future.

"As a set of protocols it has stagnated to a degree," he told us.

"Microsoft and Google have done some interesting things, but they aren’t standard. We will push, in an open way, email forward. You can only do part of that work on the client-side, the other half of the story is on the server. That’s what we can tell with these services, not alone, but with our community of open source contributors who share our desire to create next generation email."

Our testing infrastructure is in the EU. It’s likely that is where they will be located initially

Asked about plans to support encrypted email, Sipes told us: "We are still evaluating how we want to approach end-to-end encryption for our users. Right now, in testing, it is easy to have all mail encrypted, but the other piece is ensuring encrypted mail between users and key management. We have some ideas, but we will be engaging with our community on what they think is the best approach.

“When it comes to options around where messages are stored. We are looking at this very seriously. Right now our testing infrastructure is in the EU. It’s likely that is where they will be located initially. Options for users to choose what jurisdiction they are in is also a possibility."

"We’re ultimately going to make the decisions that most protect user data," he continued. “Thunderbird was resurrected by our vibrant open source community. It is governed by a community council, elected by and consisting of those same contributors. We are building these services, as a community, to take our values of privacy, data ownership, customization and respect for the user to new heights.

“All of this is a process of co-creation. Our hope is that people want to use it, in part, because of its values and how it is made. And that it can serve as a model for how to make good tech that’s good for people.”

Game plan

To bulk up, boost its functionality, and battle the behemoths, the Thunderbird team, which continues to operate under the Mozilla Foundation umbrella, is developing a set of web services - some branded as "Thunderbird Pro," alongside "Thundermail" - each in various stages of maturity:

• Thunderbird Appointment, a scheduling tool that lets users share a link so others can book time on their calendars.
• Thunderbird Send, a rebuilt version of the discontinued Firefox Send service from 2019, rewritten to support more direct and flexible file sharing.
• Thunderbird Assist, an unreleased optional AI service that will support local inference, hardware permitting, or privacy-focused cloud inference through partner Flower Labs.
• Thundermail, an email hosting service based on the Stalwart stack, to complement the Thunderbird client. Stalwart supports JMAP (JSON Meta Application Protocol), intended as a successor to IMAP (Internet Message Access Protocol).

Sipes says such services can be costly to run, so some folks will have to pay – a source of likely delight, one might think, for Mozilla as it contemplates how it may have to replace the millions it receives from Google if the US government's Chrome-busting antitrust remedy is approved by the court.

But Moz doesn't contribute financially to Thunderbird's development, according to Sipes, and whatever revenue the Thunderbird team generates will go straight into furthering its work.

"We currently are funded entirely by donations from users," he told The Register.

"Last year that number reached $10.3 million. Given that we have no other funding source – we do not receive any money outside our donors – our focus is making these services sustainable, not off the back of donors but of the users of those services. This also means a straightforward business model that respects our users. No ads. No data mining. Just pay for what you use.”

"At the beginning, we plan to offer these services for free to consistent community contributors," Sipes wrote in his planning note. "Other users will have to pay for access. Once we have a strong enough user base that the services appear to be sustainable, we will open up free tiers with limitations, such as less storage or the like – depending on the service."

Users also have the option to host some of these services, such as Appointment and Send, themselves.

Limits on free services like file sharing are necessary, Sipes observes, to prevent abuse.

"Thunderbird is unique in the world," wrote Sipes. "Our focus on open source, open standards, privacy and respect for our users is something that should be expressed in multiple forms. The absence of web services from us means that our users must make compromises that are often uncomfortable ones. This is how we correct that." ®

Editor's note: This article was updated to include quotes direct from Ryan Sipes.