Sending fifty identical messages from a normal account looks to receiving systems exactly like what a compromised account does. The consequences arrive quietly: your messages start landing in spam, including the individual ones you send to clients, and your domain reputation degrades in a way that takes months to repair. A marketing platform sends through infrastructure built and monitored for that purpose, which protects the reputation of your own domain.
There are legal requirements that come with the same threshold. Commercial email needs an unsubscribe mechanism that works, a physical postal address, and honest subject lines. A platform handles all three automatically and keeps the record that you honoured an opt out, which is the part that matters if anybody ever asks. Doing this manually is possible and it is a compliance risk sitting in your inbox.
The practical threshold is lower than people expect. If you are copying twenty addresses into the blind carbon copy field, you have already crossed it. Free tiers on the established platforms cover several hundred subscribers and a reasonable send volume, which is more than most first year businesses need for a year or more.
What you should not do is maintain two lists. Every platform imports contacts, and the value comes from having one list that is genuinely yours, exportable, and separate from any individual inbox. Keeping it in a personal email account means it disappears with that account.
One decision worth making early is where the list lives, because moving it later is tedious and lossy. Keep the authoritative copy in the platform rather than in a spreadsheet somebody updates occasionally, export it periodically so you hold a copy independent of any vendor, and record where each subscriber came from and when they consented. That last field is the one nobody captures at the start and everybody wishes they had, because it is what lets you prove somebody opted in and what tells you which sources produce subscribers who actually open anything.
Keep an exported copy of the list independently of the platform, since it is one of the few assets in a small business that cannot be rebuilt. A monthly export to your own storage costs nothing and protects against an account problem, a billing lapse, or a decision to move providers. The list is yours rather than the platform's, and holding a copy is what makes that true in practice.