I am looking for an email client that allows me to specify an arbitrary sender adress each time I send a message without explicitly creating a new identity. Especially when I hit reply to, it should use the recipient adress of the original email as sender. I am looking for Linux and Windows GUIs (e.g. Thunderbird) and/or Webmail (e.g. Roundcube). Open-source software is strongly preferred.
I have a domain, example.com where I catch all email. Let's say I buy something from a webshop that requires an email, I use firstname.lastname@example.org to sign up. That way once they start spamming me or leaked my email I can simply block it. Now let's say I want to contact or respond to their customer support via email. Then I would like to also use email@example.com to send a message. Currently with either Thunderbird or Roundcube I have to create a new Identity I am looking for a way to make that more simple.
Clarification: I want to keep the emails on my server. I am not looking for a service but a software that I can run either on my server and/or my computers that connects to the server via IMAP.