Go to Settings -> System mailbox, and click the red circle at the bottom right of the screen:
​

Select the mailbox type: IMAP (for Office 365 users), Gmail, or Exchange:
​

Click Next and follow the instructions on the screen. Traffit will ask you to choose your mailbox folders to be synchronized and a folder where the messages sent from Traffit will be saved. Once you provide all required information, click Save.