On SunOS 4 Exim crashes when looking up domains in the DNS that have more than 10 A records.

There are Sun library patches to fix this. It is not Exim's problem. For 4.13_U1 the patch is 101558-xx; for 4.1.3 the patch is 100891-xx. From the README: 1054748 ftp, ping dump core when connecting to a host with multiple DNS A records. An alternative is to build another resolver library - such as the ones that are part of the bind distribution - and explicitly link against those.