How are users connecting via Outlook? More specifically what SMTP port are they using, are they using SSL, TLS, or no encryption?
I would recommend having your users connect via port 587 with TLS on, or 465 with SSL on. Unless something weird was done, both of those should be able to authenticate (and not be blocked by blacklists).
That being said - I don't use blacklisting on the Zimbra box (we have a separate anti-spam appliance) but I do block all port 25 connections except from that anti-spam box and a few trusted servers in our environment.
State University of New York at New Paltz