site-function for completion from specific directory

[Eric Smith <es@xxxxxxxxxxxx>]

Oh zshellers,

This is my use case.

I want to complete the following command;
mutt -f [foo]<tab>

so that it expands nothing or the string `foo' as if they were
~/Mail/<tab> or ~/Mail/foo<tab>
respectively.

How would I write a compdef site-function for this and also
ideally for my shell function wrapper around `mutt -f'?

-- 
Eric Smith