Running one company is simple. Running five is not.

One business, one mailbox: no difficulty. Two businesses and you open a second tab. Five and half the working day disappears into switching between them.

The real problem

People assume the issue is simply "too many inboxes". It is not. Three distinct problems arrive together.

Nobody sees the whole picture

Support sees only the support address. Sales sees only theirs. When a customer writes to both, neither team knows what the other has already said, and the customer receives two different answers.

Access is all or nothing

You hand someone a mailbox password. They now have complete access to that mailbox: every historical message, the ability to delete, and the ability to change settings. When they leave, you change the password and redistribute it to everyone else.

Nothing is traceable

"Who sent the refund email to that client?" In a shared mailbox this question is close to unanswerable. Every message was sent by the same account.

Structure changes the problem

The model that works is one that mirrors the business:

Organisation
  Company (legal entity)
    Brand
      Category (Support, Sales, Billing)
        Mailbox

Once that structure exists, permissions become straightforward. Give one person an entire company. Give another a single brand. Give a third only the Billing category, even where it spans three separate brands.

Where to start

Write your hierarchy on paper first: which companies, which brands within each, which mailboxes within each brand. It takes about an hour and saves a great deal of rework, because correcting the structure later is considerably more expensive than getting it right at the start.

Then pilot with one brand. Do not migrate the entire organisation in a single day; give the team room to adjust.