What is the best way to send bulk email newsletters directly to the users inbox?

What is the best way to send bulk email newsletters directly to the users inbox? I Mean which way is the best which is not blocked by ISPs.

All the messages you will send first time will be gone in the inbox only. It depends upon the rating of user suppose you send 100 mails and out of which 20 person report them as a junk mail then your email id will be identified as sending bulk mails and thus if you will send any more mails even good one they will automatically redirected to bulk mail so dont try sending bulk mails which are of no use.