Unresolvable domains

need to accept

[dmac]

Sendmail v 8.9 - keep getting message log that one of my clients has "reject=451 4.1.8 Domain of sender address [e-mail] does not resolve"

Now, it seems to be intermittant - some of the stuff came over today - and just started yesterday - BUT I need the files and do not want to keep using different email addresses to communicate.

Short of it is, how can I turn on the accept unresolvable domains m4 macro? Very inexperienced with this. Sendmail currently uses procmail and spamassassin for filtering.

All help appreciated.

Thanks,
Donna