Why is Mailgun not sending a transactional email?

Question

Mail for domain

I'm trying to send an email from Laravel (to activate an account after registering on the service), via the Mailgun driver.
All settings are correct.
On authorized in Mailgun boxes (Authorized Recipients) the letter comes with a bang.
But here is the problem with new users, writes:

Client error response [url] https://api.mailgun.net/v3/...domain.../messages.mime [status code] 400 [reason phrase] BAD REQUEST

Has anyone experienced something similar?
To be honest, this is the first time I am contacting sending through third-party services, so the question may seem silly to someone, but how then to send transactional emails to new users?

Answer 1

To send transactional emails, you must link a bank card without fail! Only in this case letters will be sent normally to any address.