tFLOW supports two types of emails: outgoing emails (which are sent by tFLOW, like email alerts), and incoming emails which allows to execute some actions (e.g. Approve) on jobs via email.
IF your mail-server is behind the firewall, in order to allow the emails through firewall you should unblock incoming SMTP connection to the port specified in the system Settings Page > Email Settings:
The tFLOW server IP address, from where the SMTP request is originated, should be also white-listed to allow the connection.
If you do not know what the ip address assigned to your server is please get in contact with support team.