First, what you have and what you are paying for. There is usually a licence nobody uses and a plan that does not fit. That review often pays for the work.
Then the domain: mail records set up properly so your email is trusted, your own domain on every address, and the old mailboxes migrated without losing history. This is the part that fixes the junk-folder problem.
Then the accounts themselves — one login per person, multi-factor authentication turned on, shared files where they belong rather than on a desktop, and a documented way to close an account the day somebody leaves.
This normally sits alongside ongoing business support and a look at the network, because email problems and network problems are frequently the same problem.