Do you want to save this new message?

What happened

You tried to send a message, and your mail preferences are set to ask whether you want to save the message in your Sent view.

What you can do

Click Yes to send and save the message, No to send the message without saving it, or Cancel to go on editing the message.

If you do not want to see this prompt when you send messages, click File > Preferences (Notes Basic client users, File > Preferences > User Preferences; Macintosh OS X users, Notes® > Preferences), click Mail, and select a different option under Save mail that you send.