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Thunderbird for Android / K-9 Mail: March 2024 Progress Report

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If you’ve been wondering how the work to turn K-9 Mail into Thunderbird for Android is coming along, you’ve found the right place. This blog post contains a report of our development activities in March 2024. 

We’ve published monthly progress reports for a while now. If you’re interested in what happened previously, check out February’s progress report. The report for the preceding month is usually linked in the first section of a post. But you can also browse the Android section of our blog to find progress reports and release announcements.

Fixing bugs

For K-9 Mail, new stable releases typically include a lot of changes. K-9 Mail 6.800 was no exception. That means a lot of opportunities to accidentally introduce new bugs. And while we test the app in several ways – manual tests, automated tests, and via beta releases – there’s always some bugs that aren’t caught and make it into a stable version. So we typically spend a couple of weeks after a new major release fixing the bugs reported by our users.

K-9 Mail 6.801

Stop capitalizing email addresses

One of the known bugs was that some software keyboards automatically capitalized words when entering the email address in the first account setup screen. A user opened a bug and provided enough information (❤️) for us to reproduce the issue and come up with a fix.

Line breaks in single line text inputs

At the end of the beta phase a user noticed that K-9 Mail wasn’t able to connect to their email account even though they copy-pasted the correct password to the app. It turned out that the text in the clipboard ended with a line break. The single line text input we use for the password field didn’t automatically strip that line break and didn’t give any visual indication that there was one.

While we knew about this issue, we decided it wasn’t important enough to delay the release of K-9 Mail 6.800. After the release we took some time to fix the problem.

DNSSEC? Is anyone using that?

When setting up an account, the app attempts to automatically find the server settings for the given email address. One part of this mechanism is looking up the email domain’s MX record. We intended for this lookup to support DNSSEC and specifically looked for a library supporting this.

Thanks to a beta tester we learned that DNSSEC signatures were never checked. The solution turned out to be embarrassingly simple: use the library in a way that it actually validates signatures.

Strange error message on OAuth 2.0 failure

A user in our support forum reported a strange error message (“Cannot serialize abstract class com.fsck.k9.mail.oauth.XOAuth2Response”) when using OAuth 2.0 while adding their email account. Our intention was to display the error message returned by the OAuth server. Instead an internal error occurred. 

We tracked this down to the tool optimizing the app by stripping unused code and resources when building the final APK. The optimizer was removing a bit too much. But once the issue was identified, the fix was simple enough.

Crash when downloading an attachment

Shortly after K-9 Mail 6.800 was made available on Google Play, I checked the list of reported app crashes in the developer console. Not a lot of users had gotten the update yet. So there were only very few reports. One was about a crash that occurred when the progress dialog was displayed while downloading an attachment. 

The crash had been reported before. But the number of crashes never crossed the threshold where we consider a crash important enough to actually look at. 

It turned out that the code contained the bug since it was first added in 2017. It was a race condition that was very timing sensitive. And so it worked fine much more often than it did not. 

The fix was simple enough. So now this bug is history.

Don’t write novels in the subject line

The app was crashing when trying to send a message with a very long subject line (around 1000 characters). This, too, wasn’t a new bug. But the crash occurred rarely enough that we didn’t notice it before.

The bug is fixed now. But it’s still best practice to keep the subject short!

Work on K-9 Mail 6.802

Even though we fixed quite a few bugs in K-9 Mail 6.801, there’s still more work to do. Besides fixing a couple of minor issues, K-9 Mail 6.802 will include the following changes.

F-Droid metadata

In preparation of building two apps (Thunderbird for Android and K-9 Mail), we moved the app description and screenshots that are used for F-Droid’s app listing to a new location inside our source code repository. We later found out that this new location is not supported by F-Droid, leading to an empty app description on the F-Droid website and inside their app.

We switched to a different approach and hope this will fix the app description once K-9 Mail 6.802 is released.

Push not working due to missing permission

Fresh installs of the app on Android 14 no longer automatically get the permission to schedule exact alarms. But this permission is necessary for Push to work. This was a known issue. But since it only affects new installs and users can manually grant this permission via Android settings, we decided not to delay the stable release until we added a user interface to guide the user through the permission flow.

K-9 Mail 6.802 will include a first step to improve the user experience. If Push is enabled but the permission to schedule exact alarms hasn’t been granted, the app will change the ongoing Push notification to ask the user to grant this permission.

In a future update we’ll expand on that and ask the user to grant the permission before allowing them to enable Push.

What about new features?

Of course we haven’t forgotten about our roadmap. As mentioned in February’s progress report we’ve started work on switching the user interface to use Material 3 and adding/improving Android 14 compatibility.

There’s not much to show yet. Some Material 3 changes have been merged already. But the user interface in our development version is currently very much in a transitional phase.

The Android 14 compatibility changes will be tested in beta versions first, and then back-ported to K-9 Mail 6.8xx.


In March 2024 we published the following stable release:

There hasn’t been a release of a new beta version in March.

1 responses

Anonymous wrote on

Ah, I really like these reports.

I recently installed K9 in a new phone and I was surprised about receiving push notifications very late or not receiving them at all.

I checked it and yeah, alarms and reminders permission was not granted. I just checked it, let’s see if I start getting notifications in my new phone too!


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