I understand you are conserned that Bounce Verification may affect your Partner's email. This will depend on how the Envelope Sender is built:
- If they send as user@yourdomain.com: The emails will bounce back to your ESA, but those bounces will come from a variety of IPs. The ESA will have no way of knowing they are related to your Partner.
There are workarounds using Filters & custom headers (example: http://tools.cisco.com/squish/Cd71E) , but you would need to know details about every mailing that your Partner was planning to send on your behalf so you could create those Filters in advance.
There is an exception to this situation: if the Partner sends all the messages to your ESA for delivery to the final Recipient. Bounce Verification will be tagged on these messages so your ESA can handle the bounces appropriately.
- If they send as yourorg@theirdomain.com: The bounces will go back to thier sever and be processed there. This is considered 'best practice' as it allows the Partner to update their mailing lists; when bounces go to you, they get no feedback on thier mass mailings.