The HTTP 500 error doesn't tell us much, it is a "generic" HTTP error.
The Email Users BCC Limit setting (Dashboard > Settings > Email Users BCC Limit) is where I would start looking. How many users are you trying to send at one time? If you you have a lot and haven't set a BCC Limit, there is a good chance you're overflowing the max_input_vars limit.
Need some more details to look at this further. A long time ago a site our swim team used was a GoDaddy Windows IIS server but we moved it to their Linux platform because we had a number of email problems. That was almost 10 years ago so I am not sure I would consider that a valid data point.