compctl -u -x 's[+] c[-1,-f],s[-f+]' \ -g '~/Mail/*(:t)' - 's[-f],c[-1,-f]' -f -- mail
This is to be interpreted as follows:
If the current command is mail, then
if ((the current word begins with
+and the previous word is-f) or (the current word begins with-f+)), then complete the non-directory part (the ‘:t’ glob modifier) of files in the directoryif the current word begins with
-for the previous word was-f, then complete any file; elsecomplete user names.