Here is simple stuff need to create.
1. Create web service in PHP which accept following parameter
GetEmailParameter(FromEmail, ToEmails(Multiple with comma separated, Subject, MessageBody)
Now, here ToEmails will have multiple email address so write logic to send email separately one by one with for loop or array. Once email sent, it should respond "EmailSent".
2. Create webform in asp.net with FromEmail, ToEmails(Write multiple email with comma), Subject, Message with Submit button.
When all fields entered and hit submit button, it should call that PHP web service and pass all data and upon getting response, show message on web form.
I have budget $20 for this only as its just useless sample only.