Special characters in Mails get displayed wrong

I’m using RT 3.0.10 and it works really fine expect one thing:

If an incoming mail is encoded as ISO-8859-1 and includes special
characters like the german umlauts, they are displayed wrong.
I think it is because the conversion from ISO-8859-1 to UTF-8 fails
somehow, as mails already sent in UTF-8 are displayed correctly.

Thanks in advance,
Stefan Maier