Generally speaking, you want your email to arrive between 10AM and 2PM.
Here’s why: people tend to think of email that arrived the same day as “fresh” email. You want to give your recipients plenty of time to find (and read) your message before the day ends. So send your mail early. If you send your message late in the day, chances are it won’t get read that day. The recipient won’t see the message until the next day, and by that time, your message is no longer same-day “fresh” email. It’s old email that’s more likely to be deleted.
Also, don’t send emails before work hours begin. Why? Because you don’t want your email to be buried under a list of emails that the recipient has to dig through just to get the day started. It’s better to send during working hours in the morning.
The bottom line? Send your emails between 10AM and 2PM.
That catches nearly everyone during work hours.