According to this documentation, there is nothing stating if the delimiter can be multiple characters or a string. Can someone confirm if it is possible to use a string as the delimiter? Perhaps something like "-----"?
After reading the source code it looks like that variable eventually gets used as an int. So the delimiter can only be a single character.
However, since Postfix 2.11, there can be multiple delimiters:
When the recipient_delimiter set contains multiple characters (Postfix 2.11 and later), a user name or .forward file name is separated from its extension by the first character that matches the recipient_delimiter set.
Source: http://www.postfix.org/postconf.5.html#recipient_delimiter