Async function with slow web reference(web service) slowing down webpage
There's not enough information here to tell you what's wrong, but I'll tell you what I would do differently regardless.

Manually spawning threads is something you should avoid doing, especially if there's a chance that thread is just going to wait on a lock.  As the remote service gets slower and your app gets busier, you'll have more and more idle threads taking up resources.  You can make a single mail thread that has new params passed to it (in a thread-safe way).