Do not send e-mail from un-monitored e-mail IDs and rub it in by calling them donotreply@ or similar.
6. Logs are you friends, log all e-mail send actions in your web application.
7. Guard your reputation against joe-job accusations by using one or more sender identification/authentication/reputation frameworks like SPF and DKIM
8. Make sure you have DNS reverse records for your MTA.
9. Never use a bulk mailing software from your home DSL connection
10) Most indian companies(job sites, mutual funds, banks, online travel companies with a few notable exceptions, credit card companies and most of the social networks) don’t give a damn if a customer doesn’t want to receive their e-mails or SMS’es. Don’t be like them. Differentiate yourself
11) Add ‘a one click un-subscribe’, and ability to ‘delete’ the account of a subscriber by the subscriber herself from your web-service. A non-tedious one click un-subscribe not requiring a person to login to your site goes a long way in avoiding subscriber anger and consequent hitting of [this is spam] button in frustration. Also if one is running a social network, allowing a digest style bunching of invites/referrals/shares/ratings and other communication received over a day/week.
12) Get expert help
Try E2E Cloud to believe in it. Sign-up for a free trial here