The correct solution is to fix sendmail so it doesn't do DNS queries
until it actually needs the info for something, or to get rid of it
altogether. Various incorrect solutions could include making your own
copy of lsv_amin.c with the 'in' prefix changed to 'aa' or whatever, and
using this for your important lists. Whether it actually gives you
precedence varies from one unix to another, and it clearly won't
interrupt the job in progress (which at 30 sec per RCPT TO: could take
hours to run) to run your new request.
Eric