That server is at least 10 years old and quite custom. For example when an email is received the address is queried to an ldap and if it's a mailing list the email is sent to all the recipients of that mailing list which can be other mailing lists ! This goes on until an user address is found and then the email is delivered to the cyrus server.

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.