mayayana

If what you want to change is the mailto protocol,
do a search in regedit, or regedt32. You'll find that
mailto is similar to a file extension. It's listed as a class
under HKCR (CLASSES_ROOT). There you should see
a Shell -> Open -> Command tree, the basic format
used to set up a default "open with" setting. The default
value in the Command key is the path of the program to
handle mailto.

There's also a setting under HKCR\Protocols...., which points
to a key under HKCR\CLSID. The subkey InProcServer32
there has a default value that should also be a path
pointing to whatever handles mailto.

If you're familiar with the Registry, just do the search
and the settings you need to change should be obvious.

As far as other default settings go, I don't know how
they'd apply in an email program. With most file types,
the way it works is that Explorer encounters, say, a .txt
file.... It then looks up HKCR\.txt. The default value there
points to a file "class", which is generally "txtfile" for
..txt. Explorer then goes to HKCR\txtfile and checks
the Shell\Open\Command default value to find out which
program should be executed to open the file. So you could
change the default program for .txt files by either changing
the path in HKCR\txtfile\Shell\Open\Command, or by changing
the default value in the HKCR\.txt key to something other
than txtfile, like maybe Bills Text Program File. You'd then
create a key named HKCR\Bills Text Program File and set
up a key tree there with Shell\Open\Command, with a default
value in the Command key that points to BillsEditor.exe.

With email there isn't really a default file extension,
so the file extension setting may not be relevant.