Why is important for a business to secure its email system?
Email is not secure. It is not important for a business to "secure" its email system; it is just infeasible (by design).
Now this raises the question of how business agents will talk to each other without fearing spying, disinformation and wanton disruption from their ill-intentioned competitors. This is a hard problem. Most businesses resort to network isolation, thus splitting the network world between an inside (where everybody is honest and nice) and an outside (where roam lions and tigers and bears). For travelling agents, this isolation commonly uses a VPN.
For the same reason it's important to have a lock on your mailbox. Else anybody can pick up your mail, answer in your stead, etc.
If you have a policy where email is not secured at all by design, then don't mind putting any lock on any door.
Now if you agree that email should be secured, then you take appropriate steps, which involves either:
- Setting up your own email system, protocols for secure usage and maintenance (some users and many businesses)
- Outsource all of this (most users and many businesses)