While email marketing is still viable, it is not as highly regarded as in the past. What seems to work most effectively is group/mail/newsletter types for users who actually OPT IN for same. If your widget is that cool, they'll gladly receive in their inbox. If not, then junk filters will be applied and your widget site/letter will get a black eye. And might even rise to the level of spamhaus type isp filters. So keep that in mind, too.
Initially you need to take a database of all the email IDs you want to send. Next, you need to plan a email marketing campaign. The next step is to professional messages in the email. The content should be relevant to the message you are to convey. It should be easily understandable to the recipient.