I am grateful for your response.
I want to know please if this line “transport_email_auth_password = secret” is necessary. If yes, so please which password I have to use? How to choose it? And when I will be obliged to use it?
It’s necessary if the SMTP server you’ve configured requires authentication.
You’ll have to use valid credentials for transport_email_auth_username and transport_email_auth_password which have been given to you by the administrators of the SMTP server you’ve configured.