Foundation 6: Sender's email address is displayed incorrectly in the recipient's header

When using the mail stacks in Foundation 6, I’m having a little problem with the From Address field. I enter my email address there: service@stuttgartentrümpeln.de. When the form is completed and submitted, the recipient receives an email with the following email address in the header:
sh-720082046@eu.hosting-webspace.io instead of service@stuttgartentrümpeln.de

What am I doing wrong?

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