WordPress at a glance
filter is not described

wp_mail filter-hook . WP 2.2.0

Filters the wp_mail() arguments.

Usage

add_filter( 'wp_mail', 'filter_function_name_5101' );
function filter_function_name_5101( $args ){
	// filter...

	return $args;
}
$args(array)
A compacted array of wp_mail() arguments, including the "to" email, subject, message, headers, and attachments values.

Where the hook is called

wp_mail()
wp-includes/pluggable.php 184
$atts = apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) );

Where the hook is used (in WP core)

wp-includes/default-filters.php 219
add_filter( 'wp_mail', 'wp_staticize_emoji_for_email' );