when it was bt yahoo i was able to add reply addresses but that setting is gone
I used to forward all my email address to one bt yahoo account so they would all pop up as they came in. When i wanted to reply i just picked a reply address and it would send from there. now that function is gone i and only reply from the bt address which is no good if its work. I could set up accounts but you have to select them to check emails which again doesn't work well.
i don't see a way round it but maybe bt are gonna put that function back?
The simplest thing to do is use a mail client, you can then have as many email addresses as you want from as many providers as you want all in one place. You can then select which account to reply from, simples. There are many free ones to choose from such as Thunderbird, EmClient, Claws mail, Blue Mail or W10 built in mail app etc etc