Virtual-Identity

Thunderbird can do this with some configuration for replying to emails - set the pattern of the "Reply from this identity when delivery headers match" value of Account Settings. It's not as good as the addon Virtual Identity used to be, but it works for me 85% of the time (see https://github.com/absorb-it/Virtual-Identity/issues/22#issu...). For new messages you still need an extension like Identity Chooser or to manually edit the from header before sending.

You could have solved the second problem by storing the email with the login and password in a password manager. Maybe they were not a thing yet when you started this experiment. The third problem is more serious. I use Thunderbird. I googled and there are a couple of addons that makes it easy to edit the from address without having to create new Thunderbird identities. https://github.com/absorb-it/Virtual-Identity https://freeshell.de//~kaosmos/index-en.html#editsender Both are somewhat unsafe, one because of the site certificate, the other because of the download site. The first problem looks like a showstopper tough.