Connect Fastmail to Sickle

Fastmail does not let mail apps sign in with your account password. Instead, you create an app password with mail access — a separate password you can revoke on its own — and give that to Sickle. Sickle already knows Fastmail's server settings, so there is nothing else to fill in.

Add the mailbox once, on the web or in the Mac app, and it shows up in both — and in every agent you have connected to Sickle.

Before you start

• A Sickle account. If you do not have one yet, start a free trial.
• A Fastmail plan above Basic. Basic does not include access from mail apps, and cannot create app passwords.
• The address you signed up to Fastmail with. Fastmail only accepts that one for apps; an alias or a custom-domain address you added later will not sign in.

Create an app password

1. In Fastmail, open Settings → Privacy & Security. The direct link is app.fastmail.com/settings/security/apps.
2. Under Connected apps & API tokens, click Manage app passwords and access, then New app password. Fastmail may ask for your password again.
3. Name it Sickle and keep the default access, Mail, Contacts & Calendars.
4. Click Generate password and copy it. Leave the page open until Sickle has connected — Fastmail shows the password only once.

Fastmail's own instructions are in App passwords.

Connect on the web

1. Sign in at sickle.app and open Mailboxes.
2. Click Connect mailbox and choose Fastmail.
3. Enter your Fastmail sign-up address and paste the app password.
4. Click Connect. Sickle signs in to Fastmail to check the password before it saves anything; you see Checking… while it does.

Connect in the Mac app

1. Click + at the bottom of the sidebar (Add mailbox). On first launch, the onboarding screen shows the same choices.
2. Choose Sign in to Fastmail (⌘6).
3. Enter your address and paste the app password. Generate password opens Fastmail's app password settings if you skipped the step above.
4. Click Sign in.

Check it worked

The mailbox appears in the list on the Mailboxes page with the provider Fastmail. If you have an agent connected, ask it "Which mailboxes can you see?" — the new one is there, with nothing to reconnect.

Troubleshooting

• "Authentication failed" or "The mail server rejected these credentials". Check you used the address you signed up with, not an alias, and the app password rather than your account password or a two-step code.
• It still fails with the right address and password. Check your plan: Basic accounts cannot use mail apps.
• It worked, then stopped. The app password was deleted in Fastmail. Create a new one; the Mac app shows a Can't sign in notice where you can enter it.

Disconnect

On the Mailboxes page, click Disconnect next to the mailbox. To cut Sickle off on Fastmail's side as well, delete the Sickle app password under Settings → Privacy & Security.

Next: connect your agent

Your Fastmail is ready for any agent you connect to Sickle. Pick yours: Claude, Claude Code, Codex, Cursor or OpenClaw.