I know that I can sign up to a service using firstname.lastname@example.org, the question I have is how can I send an email from this same address?
Yes. Add it the same way you'd add any other email to send from:
- Click on Settings in the upper right.
- Click on Accounts and Import.
- In the "Send mail as" section click "Add another email address you own".
- Add the "plus email" and finish the setup.
- Compose an email and select the custom "plus email" in the From dropdown.
It is officially not supported but there is a workaround using the "send mail as"-feature:
You need to go to the
Settings tab and click
Accounts and Import. Click there
Sent mail from another address and enter your plus e-mail address. Click
Next Step and the plus mail address is ready for use. You need to do this for every address you want to use.