Error when saving a filter: Email address pattern 'example@domain.com, *@domain2.com' contains invalid characters

When attempting to create a filter (as outlined in Knowledge Base Article Essentials Filters: Expanded Overview) for an organisation or user with multiple sender/recipient email addresses or domains, you may receive an error after clicking the 'Save Filter' button:

Email address pattern 'example@example.com, *@example2.com' contains invalid characters

To resolve this error, in the 'Rule Narrative' section at the bottom of the Filters page, click on the 'IS' operator in the rule - the operator will cycle through a number of states - IS, IS NOT, IS ANY OF, IS NONE OF. In this instance, as there are multiple email addresses/domains being used in the rule, ensure you have selected the operator 'IS ANY OF' or 'IS NONE OF' (as appropriate to your rule).

You should now be able to save your filter.