'FROM' sender field

MEA00 wrote:

How to change the 'From' sender field email address when using two emails?

When you set up Mail Preferences correctly your new mail will have a drop down available if you hover your mouse over the "From" filed to make you selection.

to make this possible add the email address to the list appropriate of the account—

Mail>Preferences>Accounts>Account information

Then compose preference set the send from "Appropriate email"— if responding to an incoming email from emailOne the new message responds from emailOne as the default with no interaction required. Put your dominant email address as the first one.

Mail User Guide for Mac - Apple Support

You can also use the "Reply-to" field. Changing the "From" address also requires that you change the server to match the address. But you can set anything you want as the "Reply-to" field.