I hate to be the person to bring this up, but.... do you own the complete internet messaging infrastructure you intend to run a performance test across? If not, do you have the written permission of the owner(s) of the infrastructure that you do not own to execute a performance test?
If the answer to either question is "no" then it's time to backup and make sure you and your partners are covered legally in your efforts before you even script your first business process.
A load test against a site across resources that you do not own or have written permission to performance test constitutes a denial of service attack and can be used to seek damages against your organization. Read your Mercury licence agreement, there is a very good reason why the testing across a WAN and against sites you do not own or have permission to test against is prohibited by the license.