Sending is paused because SMTP prevents MailPoet from delivering emails

1. Mailpoet version 3.3.6
2. Browser and operating system are Safari 11.0.2 and MacOS.
3. The site is not public site and are using the plugin My Private Site. Hosted by FS-Data.
4. Screenshot can be submitted
5. Description: Our service provider FS-data has checked that the configuration and SMTP-settings in Mailpoet looks correct. During evaluation and testing of Mailpoet the subscription service worked out the first 2 e-mails notifications. But at the 3:d try we received the following error message and we can not find out why. We are not using any other SMTP-plugin which could cause a conflict.

Error message:
Sending is paused because SMTP prevents MailPoet from delivering emails with the following error: Expected response code 250 but got code “550”, with message “550 5.7.1 Client does not have permissions to send as this sender
Check your sending method settings.

6. We would really like to use this plugin as it seems to fulfill our needs ??