Any entry into the Black-/Whitelist using special characters might cause incorrect behavior of the Black-/Whitelist in general. Special characters might be interpreted by the system as a line break, splitting the entry into two separate entries, being interpreted on their own.
With the current version of the Control Panel, there is no feedback when using forbidden character, and the system will allow to save such entries. Hornetsecurity is aware of this issue and will adjust the behavior with a future update.
To prevent this issue, no special characters should be used for any Black-/Whitelist entry. This includes any characters, except @ for the mail Black-/Whitelist.