When I am busy to reply to an email or when I don’t have the answer to a query, I will promptly acknowledge it and give a time frame when I will get back to the sender.
Then, I move the email to a folder labeled “Action”.
My “Action” folder requires my action. I usually go to this folder twice or thrice a day to ensure I have responded to all my outstanding emails.
This is the method I use and it works well for me.
With Outlook, we have the reminder features which also assist us in following up with our recipients.