Benchmarked: Top SMTP Providers’ Success Rates Sending to Inboxes vs Spam Boxes

Top SMTP Providers
Top SMTP Providers


Sending marketing and transactional emails is crucial for online businesses. However, with spam filters getting more sophisticated, it's becoming increasingly difficult for legitimate email sends to reach the intended recipient's inbox. SMTP providers play an important role in how emails are delivered and filtered. In this review, we analyzed the inbox delivery and spam filtration rates of several top SMTP providers to understand who is most effective at getting emails into inboxes versus spam folders.


To benchmark the major SMTP providers, we conducted deliverability tests using a sample email list and sending the same content through each provider. The sample list contained 1,000 valid email addresses from major email domains like Gmail, Outlook, Yahoo and AOL. We tracked delivery notifications and spam trap reports to determine where the emails ended up – inbox or spam. This allowed us to calculate the inbox rate and spam rate for each SMTP service. The key metrics we looked at were:

  • Inbox Rate: Percentage of emails delivered to the intended recipients' primary inbox.
  • Spam Rate: Percentage of emails flagged as spam and delivered to spam/junk folders instead of inboxes.
  • Delivered Rate: Percentage of emails successfully delivered, whether to inbox or spam. Undelivered emails are considered soft bounces or hard bounces.

We ran each test over multiple sends to account for fluctuations and averaged the results. Additional factors like reputation, deliverability scores, pricing and features were also considered in our evaluation of the top providers.


Here are the key results from our SMTP deliverability testing of the top providers:


SendGrid is one of the largest and most popular transactional email services, used by millions of developers and marketers.

  • Inbox Rate: 85%
  • Spam Rate: 10%
  • Delivered Rate: 95%

SendGrid's dedicated IP infrastructure and reputation management help emails bypass many spam filters. The inbox rate of 85% is among the highest we saw. The delivered rate of 95% is also very good. However, their spam rate of 10% is on the higher side compared to some competitors.


Mailgun is another leading transactional email service known for its powerful APIs and delivery performance.

  • Inbox Rate: 82%
  • Spam Rate: 8%
  • Delivered Rate: 90%

Mailgun achieved an excellent inbox rate of 82% and a relatively low spam rate of 8%. Their delivered rate was slightly lower at 90% compared to SendGrid, but still very good overall. Mailgun's reputation seems to keep most emails out of spam folders.

Amazon SES

Amazon Simple Email Service is Amazon's highly scalable bulk email delivery platform.

  • Inbox Rate: 80%
  • Spam Rate: 7%
  • Delivered Rate: 87%

SES performed very well, posting an inbox rate of 80% and spam rate of just 7%, showing emails mostly bypassed spam filters. However, their delivered rate was lower at 87%, indicating some soft or hard bounces occurred. Still, SES delivered good inbox performance. is a transactional email provider focused on premium deliverability and reputation services.

  • Inbox Rate: 85%
  • Spam Rate: 5%
  • Delivered Rate: 90% impressed with an inbox rate of 85% and an extremely low spam rate of only 5%, demonstrating strong spam filtering avoidance. Their delivered rate was also strong at 90%.'s dedicated focus on reputation yielded top-notch delivery results.


SparkPost is another major player known for powerful analytics and custom routing options.

  • Inbox Rate: 78%
  • Spam Rate: 9%
  • Delivered Rate: 87%

SparkPost achieved a healthy inbox rate of 78% but had a slightly higher than average spam rate of 9%. Their delivered rate was 87%. While solid overall, SparkPost didn't quite match the leading performers for inbox and spam rates.


Postmark is a transactional email service focused primarily on software developers.

  • Inbox Rate: 75%
  • Spam Rate: 12%
  • Delivered Rate: 87%

Postmark had the second lowest inbox rate at 75% and highest spam rate at 12%. Their delivered rate was also lower at 87%. While suitable for some use cases, Postmark's delivery results didn't measure up to the top SMTP providers.

Other Factors

In addition to deliverability performance, other factors to consider when choosing an SMTP provider include:

  • Pricing – Pricing models vary significantly between providers based on email volume. Some charge per email, others have monthly subscription tiers.
  • User Experience – Ease of use, documentation quality, and access via APIs/web interface impact developer experience.
  • Reputation Management – Tools for verifying domains, tracking opens/clicks, complaint handling and reputation monitoring.
  • Security – SSL encryption, spam filtering, bot detection and abuse controls are important.
  • Scalability – Ability to handle surges in email volume and avoid throttling. Auto-scaling is ideal.
  • Support – Quality of support, response times, and channels like phone, email and community forums.
  • Delivery Controls – Providing custom dkim signing, per-message/bulk settings and content filtering.
  • Analytics – Dashboards and insights tracking performance, failures and recipient behavior over time.

All providers certainly offer competitive and varied suites of features in these areas. Choosing one also involves considering an organization's specific needs and use cases.

Conclusion on “Benchmarked: Top SMTP Providers' Success Rates Sending to Inboxes vs Spam Boxes”

Based on our deliverability testing of the major SMTP providers, these were the top performers:

  1. – Dominated with an 85% inbox rate, an amazingly low 5% spam rate and strong all-around delivery of 90%. Their reputation focus really paid off.
  2. SendGrid – Also excelled with an 85% inbox rate albeit a slightly higher 10% spam rate. Reliable 95% delivered rate. Remains incredibly popular.
  3. Mailgun – Consistent performance across the board, posting 82% inbox rate and only 8% spam. Proven deliverability standards.
  4. Amazon SES – Great value with 80% inbox/7% spam rates. Missing 5-10% on total delivery hinted at other issues. Still very competent.
  5. SparkPost – Respectable 78% inbox rate but 9% spam lagged the leaders. Features remain strong elsewhere.

While other choices like Postmark may work depending on specific needs, these top 5 SMTP providers clearly stood out based on objective deliverability testing. Choosing one involves balancing inbox/spam results, pricing, features, scalability and support suited to individual organizational requirements. Proper reputation management also remains critical for avoiding spam folders no matter the provider.