Have your e-mails already been flagged as spam, although you’ve sent a seemingly legitimate proposal to your client? Have you ever wondered why the efficiency of your newsletter campaigns suddenly dropped down? In both cases you deal with a problem which is harder to get done with than you think it is: bulletproof e-mail delivery. The main problem with undelivered mails is that both sides — sender and recipient — don’t really know what happened. Was the e-mail sent? Is the task done? Was the e-mail delivered? Most recipients will never know that an e-mail flagged as spam was sent to them — they just don’t receive the e-mail. And most senders will never know that an e-mail flagged as spam wasn’t delivered — they just don’t get a response. This article suggests over 20 bulletproof techniques, best practices and related services you can use to ensure best e-mail and newsletter delivery rates.