The plugin worked without a problem. Without changing the settings (probably after some update, but I didn’t catch it) it stopped sending newsletters.
As I test the shipment in the SMTP configuration, I get the message:
The email can not be sent: Expected response code 250 but got code “550”, with message “550 5.1.8 Sender address rejected”
I tried to transfer to MailPoet Sending Service – I went through the whole registration process, but unfortunately I don’t receive newsletters (they don’t exist in the spam folder).