Can shows ability, possibility, or permission. Will shows intention, prediction, or willingness. In a message, I can join says you are free; I will join is a commitment. When both are true, will simply sounds more like a promise.
Same message, two modalities
I can join the call at noon. You are saying you are free then. I will join the call at noon. You are promising to be there. If attendance is expected, will fits better.
We can offer a discount. The option exists. We will offer a discount. You have decided to do it. In a customer message, will reassures; can keeps the tone open.
She can finish the draft by Friday. This reads as a judgement about her ability. She will finish the draft by Friday. This is your prediction or her stated commitment. Both are fine; only you know which you mean.
Can you review this tomorrow? A friendly request, very common in emails. Will you review this tomorrow? This asks about willingness, so it can sound slightly more pointed. When in doubt, can is the safer ask.
Where one word wins outright
Asking permission is where can usually wins and will loses. Can I leave early? is standard in everyday messages. Will I leave early? sounds like a question about the future, not a request for approval.
In very formal or unfamiliar settings, can I can read as casual, and will cannot step in. Writers often switch to May I leave early? instead. For informal notes, though, stick with can.
So the decision is simple: if the message is really asking approval, keep can with peers and friends, and reach for may with a senior reader or in professional contexts.