Approve or disapprove the release

The same situation mostly happens with most of us. I think the most important thing here is QA setup similar to Production environment. This is the right time to highlight the risk factors which can cause the release unsuccessful b/c of unavailability of proper testing environment. This is only one side which can be seen this time, that you can't test the email sending at the time of order placement. What about in next requirement list or wish list, if an other option is require by user which also contain this auto email sending option. What you will do next time?

If the testing environment can't be setup as production environment
Sit with your project management team (top players)...keep the situation in front of them...get the things document (not only risk factors but your suggestion also)....discuss with them....confirm more about white box testing and/or unit testing for complete order placement code...give your standing and take the mutual decision.... and document them against your (QA) response.

Its definitely better to get things document b/c this can only be your evidence if things goes worse. With my experience I always try t