email - Set Thunderbird "from" address by incoming "to" address

I have configured my email server to send all email to my mailbox. So x@example.com and y@example.com go to one mailbox.

Every forum, registration, and guy get their own address for sending me emails so I can deliver their messages to /dev/null if anyone starts spamming. That’s the working setup.

Now the problem: If I reply to a message, then Thunderbird always sets my default Identity as the sender. I know I can add additional identities, but I don't want to add every address.

How can I configure Thunderbird so that when an email is sent to x@example.com, replies will come from x@example.com?