=== HasMailAttributeWithValueRegex

This Matcher determines if the mail contains the attribute specified in the
condition and that attribute matches the supplied regular expression,
it returns all recipients if that is the case.

Sample configuration:

....
<mailet match="HasMailAttributeWithValueRegex=whatever,<regex>" class="<any-class>">
....

Note: as it is not possible to put arbitrary objects in the configuration,
toString() is called on the attribute value, and that is the value matched against.