How do you validate a service idea when the market already has big players? i will not promote

I do automation work and recently solved an email problem for someone I work with. He was losing over an hour a day sorting his inbox and writing the same replies repeatedly. The system I built sorts mail into his own categories and pre-writes draft replies in his tone for him to review and send. It works for one person, and now I'm thinking through whether it's worth turning into a repeatable service. The challenge is the inbox-assistant space already has well-funded players. So my question is about method