I often see manuscripts and drafts that are relentless about using a comma whenever the word "And" or "But" starts a sentence. English teachers must have drilled this into us. Comma use right after but or and start a sentence is simply incorrect.
From The Editor's Blog, these examples are incorrect usage.
X But, not because of the answers I gave.
X Or, she would have to do it alone.
X For, it was a mistake right from the start.
X And, my brother needed me.
But, and, or — these are among the conjunctions we know as FANBOYS. They include the conjunctions for, and, nor, but, or, yet, so. It's completely accepted to use them to begin sentences, but that makes them connectors — and that means the clauses that follow them must refer to the sentence or phrase that precedes them.
You don't need to make a pit-stop with a comma when you're using a but or an and to start a sentence. Just get on with the connecting.