Reply from address of received message - not same as similar questions
I'll explain by means of an example.
When I send a message, both Thunderbird and my email service allows me to alter the 'from' address. I place an underscore character after my id and some chosen text.
So, I send FredCo an email message from this address: [my id]_fredco@[my domain].co.uk .
FredCo replies to the same address (and possibly stores email address).
My email delivery service recognises the address format and delivers FredCo's message to me. Because of the address format, I can see that the message is from FredCo.
(If I receive a message with that address from someone else, I know that FredCo has released my email address.)
Now when I reply to FredCo's message, the 'from' address defaults to my basic id, [my id]. And that's the problem.
I want it to default to [my id]_fredco because if I forget to change it, FredCo will record a new address, and I won't be able to detect whether FredCo has passed that email address to some other organisation.
OK, I could set up a new id for use with FredCo, but I would need dozens of similar ones - one for each person or company I contact.
Is there a solution to this problem or an add-on that will achieve this?