[Hampshire] Spamassassin misconfiguration

I've discovered SpamAssassin has been binning mail from some of my clients.
Their mail is being detected as coming directly from a dial-up IP block and the
two different RBLs it's checking for this are nudging mails over the spam threshold.

It only affects mail sent by my clients, routed as follows:

Dial-up IP --(Authenticated SMTP)--> Mauve Internet Server --(POP3)--> Office
Server with Mailscanner

The rules matched are "lastexternal", which checks that the last IP address
outside of my trusted network is not a Dynamic/Dial-up IP block.

Of course, they are, but the authenticated SMTP must trump this check.

My question is, how can I make MailScanner/SpamAssassin classify as ham mail
arriving at my (other) server over Authenticated SMTP?

Or do I need to forgo these tests given my configuration? I'd rather not try to
manage whitelists manually.

Dan