[exim] IPv6 host_lookup

I'm getting error messages and failed lookups for IPv6 clients:

"no host name found for IP address 2001:[...snip...]"

IPv4 addresses resolve correctly within exim and the logged IPv6
addresses can be correctly resolved on the command line on the mail
server host.

Do I have something configured wrong or is this a bug?
---
Steven Nikkel
steven_nikkel@???