I have the same problem, some colleague working from home with a dynIP as well, but using Outlook, not the web interface.
I could whitelist that user, so at least internal mails would come through again, but I think sending mails to other mail servers would end up in spam as well, right?
X-Spam-Status: Yes, score=4.412 tagged_above=-10 required=3 tests=[AWL=1.596,
BAYES_00=-2.599, DYN_RDNS_SHORT_HELO_HTML=0.499, HTML_MESSAGE=0.001,
RCVD_IN_PBL=0.905, RCVD_IN_SORBS_DUL=0.877, RCVD_IN_XBL=3.033, RDNS_DYNAMIC=0.1]