HI there
As I have understood your requirements, you need a app/software by which you can send email to your subscribed clients.
Well I can do this for you. To fulfill all of your mentioned requirements can use the MailChimp email service.
Solution to Your Requirements:
1) You can send any no of emails to your subscribers and that too in almost no time.
2) It will let you know if any of the sent email has delivery failure.
3) You can design email sending campaign. I men, you can design the page elements as well as the text, headings and the images.
4) You can have 2000 subscribers and you can send 12,000/month(2,000/per 24 hours). And if you need more than this, then there is a solution for that.
5) Client can unsubscribe from your list anytime.
Pls reply if need this, or let me crrect if I'm wrong.
Looking forward to hear from you.
Thanks