Its not possible that a 3rd party environment can detect how many connections users would have to your environment without that the 3rd party has access to your environment (or is part from the connection flow).
So the correct answer here is that your Exchange server is listed in the SpamHouse database with the reasons written in the SpamHouse FAQ:
Due to the vast volumes of compromised PCs spewing spam "direct-to-mx"
from dynamic domestic Internet connections, most major mail systems
choose not to accept unauthenticated SMTP mail from servers on dynamic
IPs.
Unless you use Authenticated SMTP, there is no way for a machine to
differentiate between legitimate email sent by your server from a
dynamic IP and spam mail sent by a virus on a dynamic IP next door to
yours. So, most networks make it their policy not to accept
unauthenticated SMTP email sent "direct-to-mx" from dynamic IP pools.
The Spamhaus PBL enables networks to enforce this policy.
If you're on a dynamic IP address and you absolutely need to run your
own mail server, then use your ISP's outgoing mail relay as a
'smarthost'. If your ISP does not provide an outgoing mail relay, find
a commercial smarthost provider. Such smarthosting arrangements are
very common and inexpensive; contact your ISP or a hosting company for
information. You can still accept inbound mail directly onto your
server, PBL does not affect that.
Industry best practice is to block outgoing port 25 of dynamic pools;
see M3AAWG documents (formerly MAAWG).
As you outlined you arenĀ“t using a Dynamic IP Address it might be a false positive as also mentioned by Symantec here. So you might wish to get in contact with Spamhouse directly and explain that issue and let them remove your system from the blacklist (via the SpamHouse Blocklist Removal Center) as explained here via:
A feature of the PBL is the elimination of 'false positives' with a
server-identifying and automatic removal mechanism for single IP
addresses. This allows end users with static IP addresses within a
larger dynamic pool, and legitimate mail server operators, to assert
that in their opinion their IP addresses are a trustworthy source of
email and to automatically remove (suppress) their IP addresses from
the PBL database. Safeguards are built in to prevent abuse of this
facility by spammers (and particularly by automated bots).