Problem statement
SparkPost is set up as the email provider and the test email works. When attempting to send a Passwordless email, this results in a bad request in the tenant logs with the following error:
Error sending email: Bad Request.
Solution
On the Passwordless connection, on the Settings tab, check that the domain used on the FROM address matches the domain used on the FROM address of the SparkPost provider settings (Branding > Email Provider > SparkPost Settings). This domain should also match the domain set up on the SparkPost side.