Connect Posteo to Sickle

Posteo asks mail apps to sign in with an app password — a separate password for each app that you can delete without touching your main one. Create one for Sickle, then give it Posteo's server name, which is posteo.de for every Posteo address, whatever it ends 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.
• The Posteo address you registered with — it is your username. You can look it up in Posteo under Settings → My account, in Overview.

Create an app password

1. In Posteo, open Settings → My account → Password and security.
2. Under App passwords (device passwords), click Create app password.
3. Enter Sickle as the description. Leave the App password field empty and Posteo generates one.
4. Enter your Posteo password and click Create app password now. Copy the password — Posteo keeps only a hash of it and cannot show it again.

Posteo's own instructions are in App passwords and Setting up Posteo in an email client.

Posteo's server settings

• IMAP server — posteo.de, port 993, SSL/TLS. Posteo also offers STARTTLS on port 143; Sickle does not use it.
• SMTP server — posteo.de, port 465, SSL/TLS (or 587 with STARTTLS). The Mac app sends the mail you write through it, and so does your agent when you approve a send.

Connect on the web

1. Sign in at sickle.app and open Mailboxes.
2. Click Connect mailbox and choose Other.
3. Enter your Posteo address and paste the app password. For IMAP server enter posteo.de and port 993; for SMTP server, posteo.de and port 465.
4. Click Connect. Sickle signs in to Posteo to check everything before it saves; 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 Manual authorisation (⌘7).
3. Enter your Posteo address. Under Inbox settings (IMAP), paste the app password and enter posteo.de and port 993; under Outbox settings (SMTP), the same app password with posteo.de and port 465.
4. Click Sign in.

Check it worked

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

Troubleshooting

• "Could not reach the mail server". Check the IMAP port is 993. Posteo's port 143 uses STARTTLS, which Sickle does not connect over.
• "Authentication failed" or "The mail server rejected these credentials". Use the address you registered with as the username — it is shown under Settings → My account — and the app password rather than your Posteo password.
• Posteo won't create another app password. Posteo allows five at a time. Delete one you no longer use and try again.
• It worked, then stopped. The app password was deleted. Create a new one and enter it in the Can't sign in notice in the Mac app.

Disconnect

On the Mailboxes page, click Disconnect next to the mailbox. To cut Sickle off on Posteo's side as well, delete the Sickle app password under Settings → My account → Password and security.

Next: connect your agent

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