We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/reply-forward-from-different-account/293
It's not currently possible to change the from address when writing a reply or forwarding a message you received in one of your accounts, though you can switch aliases. Setting the From address is currently only possible when composing a brand new email.
We're hoping to add support for this behavior in the future. Stay tuned!