They could've split it into smaller e-mail services in stead of designing one monolith for the planet.
I mean it's e-mail.
Its not "email"
Its all connected
M365 is a suite. Make a 365 email and you also make an account in Entra, connections to onedrive, sharepoint, teams etc etc etc
Its not "just email"
Every service becomes a monolith if it becomes popular enough.
It's probably hacked together from bits of the on-prem version. We all know what these places are like on the inside. It's not the smooth running engine they like to present it as.