Send Password Reset Email to all users at once I have a bunch (about 3000) of imported users from a website, which is currently on a redesign.
When the site is finished, I would like to send an email to all users,
saying that the site is relaunched, has a new structure ect. and therefore they have to set a new password.

I found nothing in the Craft settings, as well in the forum.

Do I have to write a plugin?
Or can I achieve this with the Sprout Email plugin?
I saw, there is a event "When a user logs in".

This could also work for me: a users logs in and gets automatically
to the a page to set a new password.
I'm not sure if the Sprout Email plugin can do anything to help you or not, but I can think of a few options that would involve simple, custom plugins.

Every user has a "Require a password reset on next login" that an administrator can set from the CP under their settings.  You probably don't want to do that manually for 3,000 users, but a simple plugin (or a simple SQL query) could set the passwordResetRequired column in the craft_users table in the database to 1.

If you wanted to send an email and include a password reset link in that email ahead of time, a plugin would change that column to 1 for all users and follow similar logic in the UsersController for getting the password reset link and include that in the email body that you'd send to every user programatically.