How can I restore a deleted email

That is a function of your email provider - not Apple (unless it is iCloud email). It will depend one whether it its an IMAPO or POP account. See the replies to:

How do I recover my emails - Apple Community

The answer depends completely on:

• Your email service provider
• Where you have directed your deleted emails to be housed (e.g., a “Trash” folder)
• Whether that email has been removed from the “Trash” folder by you should you have directed it there