Quick question (as someone who doesn't manage email): why not using an external mail provider (like mailjet) ? Privacy concerns and/or internal mails that shouldn't get leaked ?
Each user can also have many alternative addresses that map to an unique mailbox
Those mailings lists are used a lot internally and managed with internally developed tools. This make migrating to an external service difficult.
That's the classic example of some critical piece of infrastructure that nobody dare to touch because it works... until it doesn't anymore.
That's why we don't use external services to receive email. I'll consider using mailJet if setting up exim & related stuff correctly to send email in 2020 proves too painful.
I am assuming you have some budget for this.