Why does my umlaut change?
In my German emails, the umlauts are correct. But when I use an umlaut in my mailto subject, Marketo changes it to this:

I checked the HTML code, and it looks fine. Any idea how to fix this?
In my German emails, the umlauts are correct. But when I use an umlaut in my mailto subject, Marketo changes it to this:

I checked the HTML code, and it looks fine. Any idea how to fix this?
I can't reproduce this. You say "the HTML code looks fine" which suggests that your screenshot is from a mail client.
Note that letter+diaresis glyphs will need to be percent-encoded before being actually put on the wire. For example, lowercase o with diaresis must become %C3%B6. While browsers and mail clients should perform the encoding themselves when they see ö you can pre-percent-encode to ensure it will work.
(While we're on the topic, remember that you shouldn't track mailto: links.)
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