Click on the little Gear at the bottom of the left pane, then click on Manage Accounts.
Click on your Gtconline account.
Click on Change Mailbox Sync Settings.
Click on Advanced mailbox settings.
Scroll down to the bottom and make sure the settings are as follows:
Outgoing requires authentication should be checked.
Use the same username and password for sending email should be checked.
Require SSL for incoming & outgoing email should both be checked.