Required permissions for sending a message from service A to service B?

We have a windows service running as an AD user invoking an SP to send a message from service A to service B.

If I run the sp, it works fine.  If the service user runs the sp, the sp blows up with this error:

Cannot find the object "StatsMonitor" because it does not exist or you do not have permissions.

StatsMonitor is the target queue in this case.

Here is the code in the sp that blows up:

SET @Handle = NEWID()
		BEGIN DIALOG CONVERSATION @Handle
		FROM SERVICE StatsMonitorPublisher
		TO SERVICE 'StatsMonitorSubscriber'
		ON CONTRACT [http://schemas.foo.com/contracts/StatsMonitor]
		WITH ENCRYPTION = OFF;
		SEND ON CONVERSATION @Handle
		MESSAGE TYPE [http://schemas.foo.com/messages/StatsMonitor/MonitorNow](@MessageBody);
		END CONVERSATION @Handle

		if @debug = 1
		begin
			print 'StatsMonitor message sent!'
			print CONVERT(VARCHAR(MAX), @MessageBody, 1);
		end

I tried the following but it didn't seem to help:

 Please advise.

thanks