If you want to send email from your Lift app using SendGrid you need to do two things: set an authenticator and enable the javax.mail authentication flag. That second part isn't obvious.

Step by step...

Assuming your Lift props file contains the following:

# Sendgrid.com configuration: 
mail.smtp.host=smtp.sendgrid.net 
mail.user=you@example.com 
mail.password=letme1n 
 

In Boot you need to set the authenticator:

import javax.mail._ 
def optionalAuth: Box[Authenticator] = for {
    user <- Props.get("mail.user")
    pass <- Props.get("mail.password")
  } yield new Authenticator {
    override def getPasswordAuthentication = new PasswordAuthentication(user,pass)
}
Mailer.authenticator = optionalAuth

And the part that I forgot:

optionalAuth foreach { a => System.setProperty("mail.smtp.auth", "true") }