Code:
https://href.li/?https://accounts.google.com/AddSession?hl=en-GB&continue=https://mail.google.com/mail/u/0/?fs=1%26to=datagroove@protonmail.com%26bcc=%26su=hacked%26body=hacked%26tf=cm

Lets explain:
- Gmail has a limitation of 500 emails per day for each gmail account.
- Grab a few gmail accounts and use with the url above.
- The url above simply request login each time before forwarding to mail.google.com
- After the user saved the credentials on the browser, it is possible to login with just 2 clicks.
- Nice trick to send bulk emails without paying to remove the 500 emails limitation.

URL demystified...

Code:
- https://href.li/? - Clear refer
- https://accounts.google.com/AddSession?hl=en-GB& - Account login page
- https://mail.google.com/mail/u/0/?fs= - Composer url
- to= - target email to deliver
- bcc= - blind carbon copy email to deliver
- su= - email subject
- body= - email body