Hi Derek, what a quick answer!

Will test this the next day(s).

If there are further questions, can I ask here again?

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Edit: 

Oh just realized, this is to be done when you are logged in as a user. Always searched in the admin page.

If I have the same provider for outgoing mails for all local users: Do I have to set up a smtp-account for every user? 

Hi, Herbert

Yeah, you'll need to do it for every user because every user can have different settings. (Even if, in your case, they don't.) 

And what is the better strategy? Using for every user his own account for smtp, or using for all users the same?

Perhaps to avoid unnecessary traffic?

Herbert,

I can't say what the better tactic is, but if you're going to use the same SMTP account for each user, you could consider setting the SMTP service as an Outbound Gateway for the domain. You can create it at Settings > Gateways, and enable it for the domain at Domain > Options > Outbound Gateways. If you enter the auth credentials when setting up the gateway, then each user on your server will authenticate with those credentials when sending through the gateway. 

I second Jorel's suggestion. If you can use the gateway configuration, it also simplifies things for the users. When using SMTP Accounts for each user, it would only really work when using the web mail interface, and the users would need to choose to send from the SMTP account for each message. Using the Gateway, it would work whether the users are sending from the web interface or from a client, and it would be seamless to the users.