SMTP - Can't connect from Outlook/IPhone E-Mail Client

[NoNo912]

Hey community,

i've a server with Cent Os (Installed today - nothing else installed but Plesk 12.5)
I installed Plesk 12.5. on a clean, updated system.

Now there is a little problem:
I added just one email-address to check functionality.
All works fine but I can't connect from external clients.

Plesk installer has installed:
Postfix - SMTP - Connection with problems
Dovecot - IMap - Connection successful - No problems getting emails on my IPhone


I tried to connect via Telnet. (Port 25)
It was successful to connect via port 25 - but there is no chance to connect via a client to port 25.
Port 587 gives me no chance to connect to.

What is going wrong? I think there is no firewall blocking.

Hope you have a solution.

NoNo912