Using native mail App if you have set up a second mail account (from within ‘settings’) then I am pretty sure you can select either one as the ‘from’ address, when composing or replying.
My iPad has just one mail account configured and when I compose new mail or hit reply I can set “cc/bcc” as you’d expect, but no sign of any ability to change the “from” address. Whereas my iPhone has two accounts and the “cc/bcc” field also includes the word “from”. When tapped, it expands to allow me to override the other account as “from” address.
But I don’t think you can use that to set up a spurious account to spoof any old mail address, the iOS native mail setup needs to be an account one for which you have valid authentication details. You can also install different mail Apps, which might allow greater amount of tweaking, but not sure how effective such Apps are in iOS.
Might be wrong, happy to stand corrected.
