If I understand correctly, you want an exemption for a specific url or email address so it will not be tagged as Spam - a White List.
You didn' t mention which firmware you had either. It is different in 2.8 and 3.0.
In 2.8, go to Spam Filter > Email Address. Select Create New, add the address and set it to " Mark as Clear" .
In 3.0, go to Web Filter > Anti-Spam. Select Create New (you have to create a group first), select Email Address tab, add the address and set it to " Mark as Clear" .
In both versions, you need to enable the White List in the Protection Profile (Firewall > Potection Profile) under Spam Filtering. Enable E-mail address BWL check. In 3.0, you also need to select the group created, in the Option column.