I had the same "issue".
I wrote a small text in the autoresponder which contained a "ü".
In the UI, its written as "ü" but in the recieved mail (from the autoresponder) ist written as this: "Ã¼".
Coud you please say me which local is set on your system, Output of /usr/bin/locale ?
And also, the mail client name you're using.