Python 3.9 is now generally available.

google.appengine.api.mail.SendMail

Sends mail on behalf of the application.

sender Sender email address as it appears in the 'From' email line.
to List of one or more 'To' addresses.
subject Message subject string.
body Plain-text body.
make_sync_call Function used to make a sync call to an API proxy.
**kw Keyword arguments that are compatible with the EmailMessage keyword based constructor.

InvalidEmailError If an invalid email address was specified.