Apart from using the HappyFox server email address to send out replies, you can configure custom SMTP for all the outgoing emails. Setting up an SMTP ensures all your email transactions happen using your own email server and is never marked as SPAM in your recipient's inbox.
To configure SMTP,
Go to Manage >> Channels >> Email.
Choose the three dots of the email channel for which you wish to set up the outgoing SMTP and click on “Configure SMTP”.
Fill in the SMTP form with your server credentials and save it.