[exim] bounce after first 'host lookup did not complete'?

Hello,

I was very astounded to see these lines in my logfile:

2008-01-03 09:21:10 1JALK9-0008H5-JN <= support@??? H=(cath)
[217.83.41.105] P=esmtpa A=dbm_login:user@??? S=968 for
user@??? support@???
2008-01-03 09:21:10 1JALK9-0008H5-JN == user@???
R=dnslookup defer (-1): host lookup did not complete
2008-01-03 09:21:10 1JALK9-0008H5-JN ** user@???: retry
timeout exceeded

The real domain name (replaced by otherexample.com) is resolvable for
years now. There are two nameservers available and even if both
shouldn't be able to answer, exim should try it a second time. Why
doesn't exim try it again in this case (all happened within one second)?

Regards
Marten