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.
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.
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"
I was digging through Teams API calls the other day and found some references to Skype in the responses. It’s all connected indeed
Every service becomes a monolith if it becomes popular enough.