How can I reject messages with certain characters in the recipient address?

Under Incoming > Domain Settings, you can use the button Edit rejected characters to define a list of characters that must not be present in the local part of the recipient address for incoming messages. The local part is the name of a mailbox, i.e. the part before the @.

If one of the defined characters is recognized by EuropeanMX, the incoming message is rejected and not quarantined.