Send a e-mail at a later time as follows:
Click at e-mails on [New];
At the sending options (below), click on the option send on <date> at <time>.
The e-mail will be sent on the date and time you chose. An e-mail that you have sent with "delayed sending" is located under the tab "Sent". A clock behind the send date shows you that the e-mail isn't sent yet.
Modify an e-mail that isn't sent yet
Go to the tab "Sent" e-mails
Search the mail that you want to change
Open the mail and change it