The -d8.8 debugging switch causes the resolver library to be put into debugging mode (if that mode was included when that library was compiled). The ResolverOptions option (ResolverOptions) +DEBUG also turns on this debugging mode. But be aware that turning on +DEBUG will cause a large number of screens full of output to be produced by the resolver library for every DNS lookup.

If the name server returns an answer to an MX lookup, and if the answer is not an MX record or an error, sendmail will skip that host. The -d8.8 debugging switch (or the resolver library being in debug mode) then causes sendmail to print the following:

unexpected answer type wrongtype, size bytes

The wrongtype is an integer that can be found in <arpa/nameser.h >.