[exim] Umlaute and autoreply

Hello,

I am using exim with an autoreply transport for vacation messages. If
someone sends mails including Umlaute (öäü) in the subject they get
replaced with an "X" with looks not so nice. Is the a way to avoid the.
I was thinking of rewrite rules replacing the umlaute with something
like =?ISO-8859-15?Q?=FC?= but this seems quite cumbersome.
Are there any solutions to this problem?

Sincerly, Jan Suchanek