Thanks, both of you! Sorry for inadvertently posting a non-Roundcube issue to this forum, and thanks for your understanding and help. I returned the enquiry to Synology, who just replied with this:
Apologies for the confusion, on looking at your original screenshot once more, I have realised my mistake that this is of the Mail Server application and not from Roundcube as I originally thought it was.
With regards to the blacklist entries themselves, if you are aware of the mail domain address (eg. mail.synology.com) you can add it in this format, however as an alternative using synology.com as an example should also work in this instance.
To create black and white list rules:
Click the Black and White List button.
Click Create.
Specify the criteria for the rule, for example:
If you specify the IP address as "192", the messages from "192.*.*.*" will be targeted.
If you specify the email address as "admin@", the messages from "admin@domain" will be targeted.
Choose what to do with messages that fit the criteria:
Reject it: The message will not be sent from or received by Mail Server.
Accept it: The message will be accepted whether or not it was blocked by the DNSBL list.
Discard it: The message will be discarded. Mail Server will not inform the sender.
Click Finish to save settings.
Source:
https://www.synology.com/en-uk/knowledgebase/DSM/help/MailServer/mailserver_spam