The "Lightning" add-on is required for creating calendar entries in Thunderbird. In many cases, Lightning is already installed. If this applies to you, you can proceed to the section Setting up the Posteo calendar in Lightning.

Installing Lightning

1. Launch Thunderbird.
2. Click on "Tools" → "Add-ons".
3. Enter "Lightning" in the search field located in the upper right.
4. Click on the button "Add to Thunderbird" next to Lightning in the search results list.
5. Click on "Install Now".

Setting up the Posteo calendar in Lightning

1. Open Thunderbird and click on the menu "Events and Tasks" and then "Calendar" to open the calendar.
2. Click on "File", "New" and then "Calendar…".
   You can’t see the menu bar? You can display the menu bar by pressing the Alt key on your keyboard.
3. Select "On the Network" as the location for your calendar and click "Next".
4. Select "CalDAV" as the format and enter the following address in the "Location" box: “https://posteo.de:8443/calendars/john.example/default” replacing john.example with the first part of your Posteo email address. Then click "Next".
   If you use multiple calendars, each calendar will need to be set up separately. The URL of the respective calendars can be found in the webmail interface via "Settings" → "Calendar".
5. Enter a name for your calendar in the box next to "Name". Additionally, you can choose a colour for your calendar and decide whether or not you would like to receive reminders about your calendar entries in Thunderbird. Click on "Next".
6. Now enter your Posteo address as the username (in lower case) and your Posteo password as the password.
7. Tick the box next to "Use Password Manager to remember this password".
8. To finalise setup, click “Finish”.
9. Your calendar is now set up and will automatically synchronise calendar entries with your Posteo calendar. You can manually synchronise by right clicking the calendar in Thunderbird and selecting “Synchronise calendars”.

Setting up calendars from multiple Posteo accounts in Thunderbird

Some adaptations are required in order to set up calendars for multiple Posteo accounts in Thunderbird, as this is not supported by Thunderbird as standard. Proceed as follows:

1. If you have not yet installed the add-on “Lightning”, please do this first (see instructions above).
2. Open the “Preferences” in Thunderbird.
3. Click “Advanced”. Now click “General” and “Config Editor…”
4. Read the warning and then click “I’ll be careful, I promise!”
5. In the search bar, enter “calendar.network.multirealm”. A result should then appear. Double click this line. The “Value” column should then change from “false” to “true”.
6. Restart Thunderbird.

You can now incorporate calendars from various Posteo accounts simultaneously in Lightning.

Tip: In some cases it is necessary to delete all existing password entries that contain “SabreDAV” via “Settings” → “Security” → “Passwords” → “Saved Passwords” and then enter them again after restarting Thunderbird.