If it is highly important, an urgent reply would be needed regardless if it is outside of business hours. Send a message to acknowledge the email. For questions that do not have answers yet, put them in a parking lot and let them know that you will handle the matter first thing on the next business day.

If it is within business hours, prioritize the emails from the most important to the least important ones. Ignore emails that are not important. Those that do not require immediate attention can be done at a later time or send a message to give them a specific timeframe as to when you are going to tackle the issue.

If it's the first time, answer them as soon as you can then tell them that it would be better if they sent emails during business hours. If they've already done it before, then let them wait until it's business hours.