K-9 Mail has been the best-known open-source mail client on Android for years. In 2024, K-9 Mail was renamed Thunderbird Mobile — same code, same functionality, new name and new look. You’ll find the app on Google Play and F-Droid.
We use K-9 / Thunderbird Mobile ourselves — solid, platform-independent, no cloud routing.
Prerequisites
- App installed: Thunderbird Mobile from Google Play or K-9 Mail from F-Droid
- Mailbox exists
- App password if 2FA is active
Creating the account
- Open the app.
- Add account (or automatically on first start).
- Enter your email address.
- Enter the password — mailbox or app password.
- Next.
K-9 / Thunderbird Mobile tries autodiscover. With correct configuration it finds the right servers immediately.
If not:
- Choose Manual setup.
- Account type: IMAP.
- Incoming server fields:
| Field | Value |
|---|---|
| Server | mail.xaweho.de |
| Security | SSL/TLS |
| Port | 993 |
| Username | full mail address |
| Password | mailbox / app password |
- Next → Outgoing server:
| Field | Value |
|---|---|
| Server | mail.xaweho.de |
| Security | STARTTLS |
| Port | 587 |
| Authentication | normal password |
| Username | full mail address |
| Password | same |
- Next → options:
- Account name: your choice (e.g. “xaweho”)
- Display name: what recipients see as sender
- Enable push mail: yes (see below)
- Done. The account is set up.
Enabling push mail
K-9 / Thunderbird Mobile uses IMAP IDLE for push mail — an open connection to the server so new mail comes through instantly, without polling.
In the account settings → Synchronization → enable Push folders / inbox push.
Works reliably with our Mailcow. Battery usage is moderate — similar to any other app using push notifications.
Pushing multiple folders
By default, K-9 / Thunderbird Mobile only pushes the inbox. If you want to watch other folders in real time too (say, a newsletter folder), go to account settings → Folders → enable push per folder.
OAuth, Exchange — what works?
K-9 / Thunderbird Mobile speaks IMAP/SMTP with a normal password (or app password) — exactly what our Mailcow offers. You only need OAuth for Google or Microsoft accounts, not for your mailbox with us.
Exchange ActiveSync doesn’t exist with us — and you’re not missing anything: mail arrives almost instantly via IMAP IDLE (see above), and calendar and contacts run over CalDAV/CardDAV with DAVx⁵ (next section).
Contacts and calendar separately
K-9 / Thunderbird Mobile does mail only. For calendar and contacts you need a separate app:
- DAVx⁵ (free on F-Droid and Google Play): syncs CalDAV and CardDAV with the Android system.
- Setup: open DAVx⁵ → Add account → URL
https://mail.xaweho.de/SOGo/dav/<deine-mailadresse>/(DAVx⁵ automatically finds the sub-URLs for Calendar and AddressBook). - Log in with your mailbox credentials.
- DAVx⁵ runs as a background sync; the phone’s calendar/contacts apps show the data directly.
From our experience: K-9 / Thunderbird Mobile + DAVx⁵ is the most robust open-source mail suite on Android.
Profile export / import
K-9 / Thunderbird Mobile has a built-in export:
- Settings → Advanced → Export settings.
- The file is saved in the download folder.
On the target device:
- Install the app.
- Import settings → select the file.
- Account settings are taken over; you re-enter the passwords.
K-9 vs. Thunderbird Mobile — what’s the difference?
Essentially: no difference. Thunderbird Mobile is the continuation of K-9 Mail under new branding. If you still have K-9 Mail installed: you can keep it, it stays supported, or switch to Thunderbird Mobile (same codebase).
The new name Thunderbird Mobile just brings more consistent branding with desktop Thunderbird.
Frequently asked questions
Mail doesn’t arrive in real time even though push is active. Android optimizes background processes — aggressively so on some manufacturers (Xiaomi, Huawei, OnePlus). Fix: in device settings → Apps → K-9/Thunderbird → Battery, set the optimization mode to “Don’t optimize”.
Push mail only works briefly. Similar problem as above. Also check: active Wi-Fi connection (mobile data is sometimes slower for IMAP IDLE).
Multiple accounts possible? Yes, as many as you like. Separate push settings per account.
Does it work identically on Android tablets? Yes, same app, identical setup. On tablets, the UI is laid out differently (more columns), but functionally the same.
What if I want to switch from K-9 to Thunderbird Mobile? Export settings in K-9, install Thunderbird Mobile, import. Mail is on IMAP, so nothing gets lost.
Encryption with OpenPGP / S/MIME? K-9 / Thunderbird Mobile supports OpenPGP via the separate OpenKeychain app. Not S/MIME. So for very sensitive mail communication on Android, an OpenPGP setup is required.
Can I use POP3 too? Yes, pick POP3 as the account type in the setup. But: on mobile you’re making life hard for yourself — IMAP is far more practical (mail on the server, multiple devices in sync).
What’s next
First login as domain admin in the Mailcow GUI
After ordering your mail server plan, you get access to the Mailcow GUI. Here's an overview of what you see as domain admin and where to find what.
Creating and editing mailboxes, setting quotas
How to create new mail addresses as domain admin, assign storage quotas, change passwords and delete mailboxes again.
Setting up aliases and a catchall
Route several mail addresses into one mailbox — info@, kontakt@, presse@ all in a single inbox. Plus a catchall for every typo address.
Creating app passwords for mail clients
With 2FA active, Outlook, Apple Mail and friends need an app password instead of your main password. How to create them, revoke them and avoid password chaos.