This just happened. I sent an email to a hotel in Amsterdam to book an extra day after the tour. After a few hours came a replay. Sure you can book the room. Just send us your card info. I actually had already done that earlier and set up the extra day. I was alerted because the return email was from a different hotel then was on the itinerary. You have to be so careful.
Was the other hotel connected in any way to the one you wanted? Seems ominous.
For my upcoming trip, I called the Venice hotel we are staying in and reserved my room for pre and post tour. I used to send my card details to hotels in two emails. I would say in the first one that I was going to divide the numbers between the two messages.
That worked but this time I thought a phone call would give me quicker confirmation rather than waiting for an email reply.
Another reason I prefer to use my travel agent for these things.
Sorry for the delay. The other hotel was in the same town. But the correct hotel said the email was not from them.