Yes, why so surprised? How else do they send and receive messages? Postfix scales extremely well and is used by most of the Fortune 500 in some capacity - eithe

Yes, why so surprised? How else do they send and receive messages? Postfix scales extremely well and is used by most of the Fortune 500 in some capacity - either as edge servers, internal relays and as common-or-garden MTAs. The IMAP part is another server, at a guess something like Apache James or Oracle Communications Messaging Server, but possibly Cyrus (as Fastmail do) or Dovecot.