The whitelist instruction only serves to categorise spam mails. If a sender is whitelisted, their emails that were originally marked as spam will be delivered to the recipient as valid.
If an email from a sender on the whitelist is categorised as Content, Threat, AdvThreat or Rejected, it will still not be delivered.
However, with the help of a compliance filter rule, it can be set up that these e-mails are marked as valid.
With the compliance filter, rules can be set up which can be applied to both incoming and outgoing e-mails.
You can adjust the settings of the Compliance Filter in the Control Panel under Security Settings > Compliance Filter. The filter is part of the Hornetsecurity spam filter and is not charged extra.
Set the compliance filters as shown below.
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Source: https://support.hornetsecurity.com/hc/de/articles/360012205197-Einrichtung-des-Compliance-Filters