How can i change the Mailserver

Hi sebastian2086,

Thanks for visiting Apple Support Communities.

Typically, when you set up an email account on an iOS device, the server information is entered automatically. If you need to enter server information manually, you can choose "other" when adding a mail account.

This article is very helpful with setting up and configuring email accounts on your iPhone:

Add an email account on your iPhone, iPad, or iPod touch

If you need to provide a different outgoing (SMTP) mail server, you can use the steps found here:

iOS: Configuring additional outgoing mail servers

All the best,

Jeremy