Thomas,
(0) Please TRY to use search

...this has been answered at least 3 times this week.
(1) go to the SME admin panel...
Configuration / E-Mail / Delegate Mail-Servers
2nd choice text says:
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The server can deliver outgoing messages directly to their destination (recommended in most cases) or can deliver them via your Internet provider's SMTP server (recommended if you have an unreliable Internet connection or are using a residential Internet service). If using your Internet provider's SMTP server, specify its hostname or IP address below. Otherwise leave this field blank.
Address of Internet provider's mail server
....
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FILL IN YOUR PROVIDERS SMTP HERE
Voila
Reinhold