We're developing lead management software and from feedback of early customers, sending emails to marketing lists is considered as critical requirement. They gather email database at different sources like tradeshow, conferences or advertising campaigns to create marketing lists.

Now there are lot of options available to send transactional emails to subscription lists -- mailchimp, constantcontact, icontact etc. but in this case it won't be possible to verify email address by customer himself hence these marketing lists won't be allowed by these services. What would be best approach to integrate such service in lead management software? We've considered installing postfix but it has its limitations of sending certain number of emails from same IP until its blocked, and as this is going to be used by multiple clients, its more likely that large number of emails will be sent out in short time span.

Thanks!