With the few domains I own I use one as a catch-all. Kind of like a throw-away email service but only for myself. It does not receive a lot of mail and I always just create email addresses on the fly to be sent to that catch-all box. Every now and then I have to reply to an email but need to change the From field to send the reply. I have seen this in Roundcube with a plugin that allows you to type in the From field manually per each message. Do any of your web email clients support this?