folder-hook 'set send-hook' - reset to '.'?

[Charlie Allom <charlie@xxxxxxxxxxxxxx>]

Hi,

I have a feeling this is addressed time and time again, but I can't find
it in Google :)

I wish to have a default From: and .signature for 99% of my email
correspondence, except for work, which is always found in just a few
mailboxes.

How can I set this:
folder-hook . 'send-hook . "my_hdr From: Charlie Allom 
<charlie@xxxxxxxxxxxxxx>"'
folder-hook . 'send-hook . "set signature=.signature"'

        meaning, for every folder, set the send-hook to be from my personal
        account,

and this:
folder-hook =mi 'send-hook . "my_hdr From: Charlie Allom 
<charlie@xxxxxxxxxxxxxxxxx>"'
folder-hook =mi 'send-hook . "set signature=.signature_mi"'

        meaning, for =mi ONLY, set my work email From: and .signature

Currently, if I open =INBOX, and reply to someone, my personal email is
used. Then if I open =mi, my work email is used - BUT when I change back
to =INBOX, my work email is still used! how can I reset this for each
personal mailbox without specifying each on in the config file?

Cheers,
  C.

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