If the evaluation of the ESL expression does not have a result and this filter is added, the message will not be sent. It will be displayed on the Analysis page in Emarsys though. The default invalid values for this filter are "" (empty string), and the value null.
{{ contact.2|required(['-']) }}The parameter of this filter (the string - in this case) overwrites the default invalid empty string, so this string will become the new invalid one. You can still include the empty string later as well as it is possible to define more than one invalid values. For example, ([x], [y], [ ]) means that there are 3 invalid elements, the last one is the empty value.
Important: Emails prevented from being sent with therequiredfilter increase the "No content" metric. For more information on these metrics, see Email response metrics explained.