This article covers 

How to resend Email from the Email Logs, when option is grayed out?

Why is the resend email option grayed out ?

Summary 

When you try to resend a custom invoice message that was previously sent to a customer.  Sometimes, although you can filter and find this email in the Email Logs feature, the "Resend" button doesn't actually allow you to resend the invoice although it appears to be an option but is grayed out.

Solution

This would not be possible to achieve as the resend option in the email logs is available only for the failed emails and not for the succeeded emails.

If you wanted to resend the Email you can open the Email from the logs and then can use the forward option for the same.

Or you can open the invoice and use the option send email to send it.