A good boundary buys you independence

A boundary is worth drawing only if it lets the two sides change without disturbing each other. That’s the whole test. If touching one side reliably breaks the other, the line is decorative. We call this decoupled boundaries, yet highly cohesive.

This gives us a principle for where to cut: separate the part that changes for business reasons from the parts that change for technical reasons. Your rules about what an order is, when it can ship, who may approve it — those change when the business changes its mind. The fact that you store orders in Postgres, or send email through one provider or another, changes for entirely different reasons. Bind those together and every infrastructure decision can break a business rule, and vice versa. Separate them and each can move on its own schedule.