When to stop running the business off a personal Gmail
A personal inbox works fine for exactly one person — right up until a co-founder, investor or customer needs to see something in it.
Plenty of businesses start life running out of a personal Gmail or Outlook account, and for a solo founder in the first few weeks, that's genuinely fine. The problem shows up the moment more than one person needs access to the same information.
A personal inbox has no shared calendar your co-founder can see, no way to hand over an important thread when you're on holiday, and no professional domain in your email address — which is a small but real signal to anyone checking you out before doing business with you.
The switch to a proper business email platform (Microsoft 365 or Google Workspace, typically) is quick and inexpensive when you do it early. It gets considerably more painful once you've got years of email history, shared documents and client relationships tied to a personal address that only one person can access.
A reasonable rule of thumb: if losing access to your current inbox for a week would seriously disrupt the business, or if anyone other than you needs visibility into what's in it, it's time to move.