As far as I know, it's highly unusual for an agency to reach out to a writer, although it does happen, especially when you already have a fanbase or are in some other way established as a writer.
My advice would be this: Before you send them your manuscript, ask them how they got your email address and why they're interested in your writing. Reach out to that agency in a separate email (use the email address from their website, not the one they contacted you with), tell them what's going on and ask if they really sent that request. I've heard of several scams where "agents" reached out to writers and I hope you got a genuine request, but you can't be too careful.