To reproduce send an email using the Send Email action that has the following text:
Viele Grüße,
There are two umlauts in this text. Ensure that the "Message is HTML Formatted" checkbox is NOT checked. On the receiving side this ends up coming in as:
Viele Gru?e,
Changing the encoding in the action properties has no effect. It looks pretty much like you are using a Unicode to ANSI conversion here when sending the mail. ANSI is 7-bit and doesn't support umlauts.
I tracked this down to a change in behavior in the library we are using. I changed the point at which we set the encoding and it seems to work correctly now. We do not do any encoding conversion in this action, all text we pass to the library is UTF-16 and we tell it which encoding to use when sending the email. In your case you need to set the encoding to UTF-8 or one of the ISO encodings.