Hi
@cPSamuel,
We talking for remote mail servers, listed in DNS zone of domains with higher MX than primary one.
I made tests from one of the remote servers :
first sending mail to domain which have this remote mail server as MX record on its zone:

logs:
2020-05-26 11:44:25 1jdVC0-000AY2-Tu H=........ [...........]:48516 Warning: Message has been scanned: no virus or other harmful content was found
2020-05-26 11:44:25 1jdVC0-000AY2-Tu <=
[email protected] H=........... [...........]:48516 P=smtp S=233 T="test mail 1" for
[email protected]
2020-05-26 11:44:25 cwd=/var/spool/exim 3 args: /usr/sbin/exim -Mc 1jdVC0-000AY2-Tu
2020-05-26 11:44:25 1jdVC0-000AY2-Tu => ........... (
[email protected]) <
[email protected]> R=virtual_user T=dovecot_virtual_delivery_no_batch C="250 2.0.0 <kosyo...........> KAdXFenWzF6VkgAAzxBFKw Saved"
2020-05-26 11:44:25 1jdVC0-000AY2-Tu Completed
second sending mail to domain on same server that has NO MX record on its zone:
2020-05-26 11:48:57 1jdVGR-000C42-Fb H=........ [........]:48524 Warning: "SpamAssassin as ........detected message as NOT spam (-96.1)"
2020-05-26 11:48:57 1jdVGR-000C42-Fb H=........ [........]:48524 Warning: Message has been scanned: no virus or other harmful content was found
2020-05-26 11:48:57 1jdVGR-000C42-Fb <=
[email protected] H=ns.xgoth.com [212.36.24.5]:48524 P=smtp S=1788 T="test 2" for
[email protected]
2020-05-26 11:48:57 cwd=/var/spool/exim 3 args: /usr/sbin/exim -Mc 1jdVGR-000C42-Fb
2020-05-26 11:48:57 1jdVGR-000C42-Fb => kosyo........ (........) <........> R=virtual_user T=dovecot_virtual_delivery_no_batch C="250 2.0.0 <kosyo........> sA/HFPnXzF6VkgAAzxBFKw Saved"
2020-05-26 11:48:57 1jdVGR-000C42-Fb Completed
As you can see second mail pass thru SpamAssassin first one not at all...