How do I change the 'From' address in Apple Mail?

Hello all,

I just encountered an issue that seems new. When starting a new email, one of my many email addresses will populate the “from” line. When I click it in an attempt to change the email address it shows four of my email addresses and the Hide My Email options. It will not allow me to enter any other address. Anyone else having this issue?

[Re-Titled by Moderator]

Original Title: Apple Mail From addressing