‘Enable send as’ feature not working

Hi, we set up the plugin according to the instructions, Version 1.8.6.
It is working without the ‘Enable send as’ feature and is sending emails from [email protected], but when we activate the ‘Enable send as’ feature, no email is being sent (Nothing happens). The ‘send as’ permission doesn’t seem to be working. From our view everything has been configured correctly.

Any ideas for troubleshooting this?

Thank you!
Martin