1. In a networked computer system comprising at least one memory to store processor-executable instructions and at least one processing unit communicatively coupled to the memory, a method for modifying an E-mail message to integrate electronic calendar functionality, the method comprising: obtaining, at a local machine of an organizer, a draft E-mail message from the organizer to one or more recipients; obtaining, by a server via a network, calendar information depicting the availability of the organizer from an electronic calendar system of the organizer; displaying, at the local machine of the organizer, said calendar information depicting the availability of the organizer; obtaining, by the server, a first set of one or more tentative time periods for a tentative calendar entry selected at the local machine of the organizer; automatically generating, by the server, a first image depicting, at least in part, said first set of tentative time periods; automatically generating, by the server, a Uniform Resource Locator (URL) associated with said first image; automatically modifying, at the local machine of the organizer, underlying HTML of said draft E-mail message to include a tag that references said URL so as to display an image obtained from said URL; displaying, at the local machine of the organizer, at least a portion of said first image in said draft E-mail message; obtaining, at a local machine of at least one of said one or more recipients, a received E-mail message that includes said tag that references said URL; receiving, at the server, a request to access said URL from the local machine of said at least one recipient in response to opening said received E-mail message at the local machine of said at least one recipient; obtaining, by the server via the network, up-to-date calendar information depicting the availability of the organizer from the organizer's electronic calendar system; automatically modifying, by the server, said first set of tentative time periods to create a second set of tentative time periods by updating said first set of tentative time periods to include changes derived from said up-to-date calendar information depicting the availability of the organizer; automatically generating, by the server, a second image depicting, at least in part, said second set of tentative time periods; and automatically including, in a display of said received E-mail message at the local machine of said at least one recipient, a display of at least a portion of said second image depicting, at least in part, said second set of tentative time periods.