Under each user's account in SETTINGS > ACCOUNT SETTINGS > COMPOSE there is a check-box for "Enable trusted sender for webmail recipients". Enabling this will automatically add any senders to the Trusted Senders list for that user, resulting in their emails never going to the Junk E-Mail Folder.

 

To set this for all users on a domain, as the domain administrator go to SETTINGS > DOMAIN SETTINGS > ADVANCED SETTINGS > USER DEFAULTS > COMPOSE and enable it there. To propagate this change to all existing users you would go to SETTINGS > DOMAIN SETTINGS > ADVANCED SETTINGS > USER ​ PROPAGATION > COMPOSE and select it and press [PROPAGATE NOW].

 

To set this globally for all users on the Mail Server, you would go to SETTINGS > DEFAULTS > USER DEFAULTS > COMPOSE and enable it there. To propagate this change to all existing users you would go to SETTINGS > DEFAULTS > USER PROPAGATION > COMPOSE and select it and press [PROPAGATE NOW].

 

Note that this only will affect those using webmail. Users sending email using an email client or mobile device may still have recipient's replies end up in their Junk E-Mail folder if it exceeds the Antispam Filtering Weight Threshold for that action.