Thunderbird has had it for ages - you do need to pick it from the menu (“customize from address”) for completely free form. You can also set up multiple identit

Thunderbird has had it for ages - you do need to pick it from the menu (“customize from address”) for completely free form. You can also set up multiple identities to pick from for the more frequent addresses you use; I set up the 6 most used, and now only need to use the “customize..l option about once a week (compared to several a day before, like you)