Why does Outlook's message recall feature work sometimes but not all the time?
Outlook's message recall feature is designed for use within Exchange Server organizations. As it turns out, message recall works only with messages that you send in Rich Text Format. RTF is typically used between recipients in the same organization. However, after a message leaves the organization, all bets are off—depending on which kind of server is receiving the message and how you've configured your connectors, the message might or might not be sent in RTF. Even if it is sent in RTF, no guarantee exists that the receiving client will honor the recall request. As a matter of routine, many users skip recall requests to read the message being recalled on the theory that anything that needs recalling must be worth reading.