SMTP through GMail

I'm trying to send notification e-mails through my GMail account and get the following error:

3 An error occured sending the message, Error:80040207, Description: Unexpected MAIL FROM response, Last Response: 530 5.7.0 Must issue a STARTTLS command first. 24sm30374988wfc.2

This happens no matter which authentication method I choose. Other settings:

SMTP server:
port: 587
username/password are valid

Any hints?