If you are using an IMAP account, it may be configured to save sent messages on the server. Please synchronize all messages ...

If you are using an IMAP account, it may be configured to save sent
        messages on the server.   Please synchronize all messages in
        your sent items folder.  To prevent this problem in the future, you
        can configure your account to use your local Sent Items folder instead
        of one located on the server.