This is a planned behaviour if a new email account has been set up on a Symbian ^3 phone. The sent folder is never synced with the email server if the mailbox is using IMAP or POP3 for the connection. However, you can still sync the sent folder using ActiveSync.
When sending an email from your phone, it will be synced to the sent folder of the email client on your phone, but it will not be synced to your server's sent folder. Similarly, emails sent using other devices and clients are not synced to your Symbian^3 phone's sent folder.