How to write a rule for 'After sending email'? I have to write a rule for the event 'After sending email'. How can I do that?

For example there are three users user1, user2, user3. I need to enable a flag on a content type after admin sending email to those three users.
There is no Rules Event like "*after sending eMail". However, you can create a Rules Component for sending an eMail. For a sample of such Rules Component, refer to the answer to "Rule to store user&#39;s last post date and scheduled rule based 30 day reminder".

If you then "invoke this Rules component" any time a rule has to send an eMail, you basically created the equivalent of the Rules Event you're asking for. All that's left to do is to expand that Rules Component with additional Rules logic that you want to happen "after sending eMail" (... as in your question).