diary-outlook-formats is a variable defined in `diary-lib.el'.
Its value is

(("[ \n]*When: [[:alpha:]]+, \\([[:alpha:]]+\\) \\([0-9][0-9]*\\), \\([0-9]\\{4\\}\\),? \\(.+\\)\n\\(?:Where: \\(.+\n\\)\\)?" . diary-outlook-format-1))

Alist of regexps matching message text and replacement text.

The regexp must match the start of the message text containing an
appointment, but need not include a leading `^'. If it matches the
current message, a diary entry is made from the corresponding
template. If the template is a string, it should be suitable for
passing to `replace-match', and so will have occurrences of `\D' to
substitute the match for the Dth subexpression. It must also contain
a single `%s' which will be replaced with the text of the message's
Subject field. Any other `%' characters must be doubled, so that the
template can be passed to `format'.

If the template is actually a function, it is called with the message
body text as argument, and may use `match-string' etc. to make a
template following the rules above.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.