Symptoms- After removing mailman3 as a Plesk component, its services mailman3.service and mailman3-web.service
may stay loaded, active and running.
CauseBug confirmed as
PPPM-14815.
When it is fixed it will appear in our release log: Change Log for Plesk Obsidian
ResolutionAs temporary workaround, manually stop and disable the services:
#systemctl stop mailman3 mailman3-web && systemctl disable mailman3 mailman3-web