When I send emails where I delegate tasks to someone else I usually flag those emails as "waiting for" so as not to forget to remind that person about the task later. I use a blue flag for that. 

Now currently I send an email and then access the Sent folder, find the message (usually it's on the top, because it was just sent) and flag it. However, this is a bit tedious.    

It would be great to have the ability to flag a message on the editing stage, similar to how the "Remind me" option works now (an icon in the top right corner of the mail editor window). In other words, I would write an email, flag it right there in the editor and hit Send. The email will get sent and flagged so I won't have to access the Sent folder to flag it. 

Thank you.