From specification:
Parameter Name: EMAIL
Purpose: To specify an email address that is used to identify or
contact an organizer or attendee.
Format Definition: This property parameter is defined by the
following notation:
emailparam = "EMAIL" "=" param-value
Description: This property parameter MAY be specified on "ORGANIZER"
or "ATTENDEE" properties. This property can be used in situations
where the calendar user address value of "ORGANIZER" and
"ATTENDEE" properties is not likely to be an identifier that
recipients of scheduling messages could use to match the calendar
user with, for example, an address book entry. The value of this
property is an email address that can easily be matched by
recipients. Recipients can also use this value as an alternative
means of contacting the calendar user via email. If a recipient's
calendar user agent allows the recipient to save contact
information based on the "ORGANIZER" or "ATTENDEE" properties,
those calendar user agents SHOULD use any "EMAIL" property
parameter value for the email address of the contact over any
mailto: calendar user address specified as the value of the
property. Calendar user agents SHOULD NOT include an "EMAIL"
property parameter when its value matches the calendar user
address specified as the value of the property.
Example:
ATTENDEE;CN=Cyrus Daboo;EMAIL=cyrus@example.com:mailto:opaque-toke
n-1234@example.com