For reliable mass email sending, both iDealSMTP and SendGrid are excellent choices, but they cater to slightly different needs. SendGrid is generally the industry standard for high-volume transactional emails and provides superior API integration and robust, detailed analytics, which is essential for maximizing delivery success rates.
iDealSMTP often appeals more to the Grey/Black Hat side due to its flexibility with content filtering, but its deliverability can sometimes be less consistent than SendGrid's strict standards. For professional, long-term, high-volume campaigns where detailed reporting and high inbox placement are paramount, SendGrid (or Mailgun/Amazon SES) offers the better, more scalable infrastructure and value.