How do you send mass emails from ASP.NET?
For this kind of task you're better to add a bunch of jobs to a queue. Then have a thread running which pulls x number of jobs from the queue, processes them (i.e. sends the emails) and then sleeps for a period of time. This will give you web app some breathing space.
If you're using a database you can create an Email Queue table to store the jobs. I prefer to use this kind of storage over memory incase the app recycles for some reason or an exception is thrown...atleast you can then pick up from where you left off.
Generally, the process running the worker thread wouldn't be the web app itself. It would be a windows service or something similar. This might not be possible if you're on shared hosting.