Some of the above keywords specify a Lisp function to be executed.
For each keyword :foo, the Lisp variable foo is bound to
the value of the keyword while the function is executing. For example,
consider the following mail-source setting:
(setq mail-sources '((pop :user "jrl"
:server "pophost" :function fetchfunc)))
While the function fetchfunc is executing, the symbol user
is bound to "jrl", and the symbol server is bound to
"pophost". The symbols port, password,
program, prescript, postscript, function,
and authentication are also bound (to their default values).
See above for a list of keywords for each type of mail source.