Relaying by apop or smtp-auth?

[speedy66x]

Hi Guys,

excuse my english please!

Plesk offers two Relay-Modes:
APOP (SMTP after POP)
SMTP (Log-In to SMTP-Server)

I don't know whats the result after Activating both Services at 'Admin-Panel' -> 'Server' -> 'eMails'.

May the Client choose between the different kinds of Authentification or had the client to use both at the same time together?

Regards,
Daniel