This is a shortcoming of the existing quota infrastructure -- only a single email address can be specified (either via CLI or GUI, doesn't matter).  Although one would think that something like "foo[NAME].com;bar[NAME].com" should work, it doesn't.  There's a bug filed for this in engineering already.  The current workaround, as folks here have pointed out, is to use a distribution list.  We recognize that this is not optimal.  If this issue is really disruptive, then I would suggest that you also work with your SE to get a feature request filed with Product Management.  That will then affect the engineering bug in positive ways...
Hi  I want to add multiple email addresses to Quota Notification. How can i do?  Thanks
Isilon Quota Custom notification