Thank you for your post.
You do not need to remove the Spam Assassin; MagicSpam works very well in combination with a filter such as SpamAssassin because they work at different levels". MagicSpam reduces load on your server by blocking messages directly in the SMTP layer based on IP reputation and SMTP best practices (You can choose which lists and rules to implement). This lowers your load because it will block messages instead of passing them on to be processed further by software such as SpamAssassin, it also subsequently reduces network usage and disk usage for the same reasons.
Also, please note that for our MagicSpam PRO customers, we are working on releasing access to our own content filtering rules, which will work with the SpamAssassin.
Please note that MagicSpam does not work with those Plesk-API specific individual anti-spam settings directly. Messages accepted by MagicSpam are passed down to the Plesk/MTA for further processing, and those individual anti-spam settings (white/blacklist, anti-spam behavior, e.g.) would take effect at that stage.
We would suggest the MagicSpam PRO version for your needs as it offers a "FLAG' feature where detected spam messages are passed to the customer's spam folder instead being rejected right away.
Considering that MagicSpam for Plesk (Standard) rejects detected spam messages by default, detected spam messages send by a user on a Plesk-level individual setting's whitelist would be rejected, as these Plesk-level 'individual' whitelists are not used by MagicSpam.
We hope this information helps.
-- MagicSpam Support Team --