There are two ways which you can whitelist a domain or email address so any emails coming from that user or domain will not be labeled as spam and either rejected or moved into your spam folder. Follow the instructions below on how to add a email or domain into your whitelist.

Login to your webmail

The first option is to use the Spam option within webmail. 

Follow the steps below

1. Once logged in, select 'Settings' located at the top-right corner.
2. Select 'Email Accounts'
3. Click on the "Spam" tab along the top.
4. In the white list text box, enter the domain or email address. For example if I was going to enter a domain, it would be "example.co.nz". If I was going to enter a email address it would be "user@example.co.nz". Remember to replace example.co.nz and user with the actual domain and email user of the email you want to white list. 
5. Click on "Save"

The other option is to use the filters option.

1. Once logged in, select 'Settings' located at the top-right corner.
2. Select 'Email Accounts'.
3. Select 'Filters'.
4. Select '+Add'.
5. Under Filter Conditions, choose 'Select the conditions for your filter:'
6. From the drop-down menu, choose 'If any of these conditions are met'.
7. Choose Type 'From' - regex - matches - ^.+@.+.[domain]?$ (replace [domain] with the specific domain you want to whitelist, e.g., ^.+@.+.domain.com?$)
8. Under Filter Actions, select 'Move to: INBOX'.
9. Press 'Save'.

This will also do the same as whitelisting. Remember not to whitelist large email providers domains, such as gmail, hotmail or yahoo mail. You will be allowing any email, including spam email using one of these providers into your inbox.