Before using this program, you should get an access token and a refresh token from Google API. Please refer to this document for more details : https://developers.google.com/identity/protocols/oauth2
Set these env vars before running the program:
GOOGLE_CLIENT_ID=""
GOOGLE_CLIENT_SECRET=""
GOOGLE_REFRESH_TOKEN=""
GOOGLE_ACCESS_TOKEN=""go run main.go -to=recipient_email_address@domain.com