For emails that require a bit of research before a reply, I first send a quick reply telling the sender there will be a little delay — just to be polite — I then forward the email to my Todoist inbox, and put the original email into my “Action Today” folder.
If I cannot get the infomation on that particular day, it doesn’t really matter as the email will still be in my Action Today folder tomorrow as a reminder I still need to action it.
For me, anything that’s been in my Action Today folder lonfer than 48 hours is a problem and it becomes a priority to sort out. So, it would be dealt with reasonably quickly.
Hope that helps.