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Click on the little Gear at the bottom of the left pane, then click on Manage Accounts.
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Click on your Gtconline account.
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Click on Change Mailbox Sync Settings.
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Click on Advanced mailbox settings.
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Scroll down to the bottom and make sure the settings are as follows:
Incoming: mail.gtconline.com:993:1
Outgoing: smtp.gtconline.com:465:1
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.