In order to be able to send emails or the function "Registration with Welcome email" you have to use an SMTP server.
We do not offer SMTP / EMAIL services, which is why you have to use an external one.
You can then send us the SMTP details according to the documentation and we will set the settings for you.
The format of the config looks like this:
# The notifications are sent using sendmail, unless the SMTP_SERVER variable is set.
# In that case, make sure the rest of the variables are properly set.