How can I delete an email that I don’t want?

Email message or Messages messages? For email messages, move them to the Trash. On a computer, select the email, and hit the delete key. On a device, drag the email to the left, and select Trash.

For messages, on a computer, select the message in the left Sidebar, then on the right side of the message, click the X. On a device, tap Edit at the top left, then tap the circle on the left side of the message, and tap Delete.

Remove the email account from Mail or delete it from somewhere else? Change your Apple ID email address? More information would help.

Note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.

How do I trash messages that I don’t want?