A large part of my job is end user support in a corporate environment. I always do this - even if I know an issue is going to take a while due to me having to reach out to other departments/a vendor, wait for an answer, and potentially go back and forth, I always reach out to let people know I got the email, that I'm working on it, and that I'll reach back out when I know more. If possible, I also suggest workarounds/alternatives for them to use/do in the time while I'm working on the problem.