K-9 Mail Setup on Android: The Complete Guide

K-9 Mail is the open-source Android client that the Thunderbird team adopted and turned into Thunderbird for Android — whose first stable build, version 8.0, shipped on 30 October 2024. The app you install today still works exactly as it always did: it speaks plain IMAP and SMTP, so it connects to almost any mailbox once you know the ports. I set up K-9 on a Pixel against a Gmail account and a self-hosted IMAP box, and the only thing that trips people is the Gmail app-password step. Here is the whole setup — install, auto-config, manual servers, the Gmail credential, push sync, notifications, and multiple accounts.

K-9 Mail and Thunderbird for Android

K-9 Mail is the codebase that became Thunderbird for Android. The Thunderbird team took K-9 under its wing, and the first stable Thunderbird for Android release — version 8.0 — shipped on 30 October 2024. K-9 still runs and still installs, but development is moving to the Thunderbird-branded app, with a built-in import path between the two.

Before the first tap, it helps to know what you are installing. K-9 Mail has been a staple Android email client for years, and the Thunderbird project adopted it to build Thunderbird for Android, releasing the first stable version 8.0 on 30 October 2024. The two share a lineage and a settings model, and the project ships an import tool so you can carry K-9 accounts straight into Thunderbird for Android.

That matters for setup in one practical way: everything below — auto-config, manual IMAP/SMTP, app passwords, push — works the same in both apps, because Thunderbird for Android is K-9 with a new coat of paint. Learn it once here and it transfers. If you eventually move to the desktop counterpart too, our guide to migrating to Thunderbird covers that side.

Install From F-Droid or Play

Get K-9 Mail from F-Droid for a fully open-source build, or from Google Play for automatic updates — both ship the same app. Open it once and the account wizard starts on its own. The install source changes nothing about the setup steps that follow.

K-9 Mail is published in several places, and the choice is about how you like to update, not about features:

• F-Droid — the fully free and open-source route. Install the F-Droid store first, then K-9 Mail from it. Best if you avoid Google Play on principle.
• Google Play — the simplest path, with automatic background updates. Best for most people.

Both builds are functionally the same client. Open the app after install and the first-run wizard launches automatically, dropping you straight into adding an account. There is no account or sign-up to create for K-9 itself — it is a client that connects to mailboxes you already have.

Add Your Account With Auto-Config

Type your full email address and password and let K-9 attempt auto-discovery. For common providers it detects the incoming and outgoing servers and fills them in. If detection fails, the wizard hands you to manual setup, where you enter IMAP and SMTP yourself.

The fastest path is to let the app find the servers for you:

1. On the Set up your email account screen, enter your full address (e.g. you@example.com).
2. Enter your password — or, for Gmail and other providers that support it, sign in with OAuth when prompted.
3. K-9 tries to auto-detect the IMAP and SMTP servers from the domain. For mainstream providers this just works, and you can skip straight to naming the account.

Auto-config covers the big providers cleanly. Where it stumbles is custom domains, smaller hosts, and corporate mail — anything where the server names do not follow a predictable pattern. When that happens, the wizard offers a manual option, which is where the next section comes in.

Enter IMAP and SMTP Manually

For incoming mail, choose IMAP with SSL/TLS on port 993. For outgoing mail, choose SMTP on port 465 with SSL/TLS, or port 587 with STARTTLS. These are the standard secure email ports from RFC 8314. Enter your provider’s hostnames and use your full email address as the username.

When auto-config can’t guess your servers, you fill them in by hand. The encryption ports are standardised — RFC 8314 assigns IMAP-over-TLS to port 993 and SMTP submission to port 465 (implicit TLS) or 587 (STARTTLS) — so the only provider-specific parts are the hostnames:

Incoming server (IMAP)

• Server: your provider’s IMAP host (Gmail: imap.gmail.com)
• Security: SSL/TLS
• Port: 993
• Username: your full email address

Outgoing server (SMTP)

• Server: your provider’s SMTP host (Gmail: smtp.gmail.com)
• Security: SSL/TLS on port 465, or STARTTLS on port 587
• Authentication: required, same username and password as incoming

Pick IMAP, not POP, unless you have a specific reason — IMAP keeps the same mailbox state across every device, which is the whole point of reading mail on a phone and a laptop. The same port logic applies in any client; if you set up a desktop tool later, our Mailbird IMAP setup guide uses identical values.

The Gmail App-Password Step

Gmail blocks your normal password in third-party apps. Turn on 2-Step Verification, generate a 16-character app password in your Google Account, and paste that into K-9 instead of your real password. The app password is single-purpose and revocable without touching your main credential.

This is the step that breaks most first-time setups. If you enter your everyday Gmail password into K-9, Google rejects it. The fix is an app password — a dedicated credential for one app:

1. Turn on 2-Step Verification in your Google Account. App passwords require it and cannot be generated without it.
2. Generate an app password — Google produces a 16-character passcode.
3. In K-9’s password field, paste that 16-character code instead of your normal password.

Because it is scoped to one app, you can revoke it later without changing your main Google password or affecting other devices. The same mechanism applies to any client connecting to Gmail; the full walkthrough — including where the option hides — is in our Gmail app passwords guide. Many providers now prefer OAuth sign-in, which K-9 also supports; where OAuth is offered, take it and skip the app password entirely.

Folder Sync, Push, and Notifications

In the account’s folder settings, enable Push for the inbox so new mail arrives in real time over Push IMAP, choose which folders sync, and set per-account notifications. K-9 lets each account have its own notification sound and behaviour rather than one global setting.

A working account is not a finished setup — the defaults rarely match how you actually want mail to behave. Two settings do the heavy lifting:

• Push IMAP. K-9 supports Push IMAP and per-account notifications, so the server can notify the app the instant mail lands instead of polling on a timer. In the account’s folder settings, set the inbox (and any folder you watch) to Push for real-time delivery. Polling is the battery-friendly fallback if your provider doesn’t support push.
• Folder sync. Choose which folders sync and how often. Syncing every label or folder is wasteful on mobile data; limit it to the folders you actually read.

For notifications, K-9 lets you configure them per account — a different sound for work versus personal, or notifications off entirely for a low-priority address. That granularity is exactly what makes running several accounts on one phone bearable.

Signature and Multiple Accounts

Set a per-identity signature in each account’s settings, then add more accounts from the main settings menu. K-9 supports multiple accounts and multiple identities, with a combined Unified Inbox and separate server settings, signatures, and rules per account.

Finish by making each address feel like its own. K-9’s per-account settings let you set a signature appended to outgoing mail, and because it supports multiple identities, one account can send as several addresses or aliases.

To add more accounts, return to the app’s settings and add each one through the same wizard. K-9 handles multiple identities and per-account notifications, and offers a Unified Inbox that merges every account into one stream — useful when you want a single triage view, with the option to drop back into a single account when you need focus. Each account keeps its own servers, signature, and sync rules, so a work mailbox and a personal one never bleed into each other. If you protect those accounts with strong filtering, pair the setup with a Thunderbird spam-filter setup on the desktop side for the same providers.

Verdict

K-9 Mail is the most capable open-source Android email client, and setup is straightforward once you know two things: use IMAP on port 993 with SSL/TLS and SMTP on 465 or 587, and use a 16-character Gmail app password instead of your real password. It is now the basis of Thunderbird for Android, so the skills transfer directly.

Best for: anyone who wants a free, open-source, privacy-respecting email client on Android that connects to any IMAP/SMTP mailbox, supports push, and handles multiple accounts cleanly. The F-Droid build in particular is hard to beat for an entirely open stack.

Skip if: you only use Gmail and are happy in the official Gmail app, or you want a fully native desktop experience on Windows — in that case a desktop client like eM Client or Mailbird fits better, while K-9 stays the phone half of the setup.

Install from F-Droid or Play, let auto-config try first, fall back to manual IMAP/SMTP with the standard ports, swap in a Gmail app password where needed, then turn on Push and per-account notifications. Ten minutes of setup and your phone handles every mailbox you own — with a clean migration path to Thunderbird for Android whenever you want it.

Alexis Dollé

Founder & Editor

Alexis Dollé, email expert for 10+ years. Founder of Email Tools. I test every email client and utility myself, then write about them the way I’d explain them to a friend — no marketing fluff, no sponsored rankings, every claim sourced.

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Related: How to migrate to Thunderbird — move accounts to the desktop counterpart. Gmail app passwords — the credential K-9 needs for Gmail. Mailbird IMAP setup guide — the same ports on a desktop client.