For sending in Mailbox.org webmail there is a thing called alternative senders where I can add email addresses to send from. I have something similar on my Android K9 app, where it's under Manage identities where I can also add the allowed senders.
I set it up a long time ago this way and I'm unsure of I had to somehow configure Gmail to be able to send from my other account. But I have the same also for @live.com and @zoho.com as I was trying different providers. This allows me to easily reply to forwarded emails with the old email addresses.
(I don't actually know how often I send email from that address, so maybe I don't even need that, but just in case.)