{"id":9327,"date":"2020-05-07T07:44:15","date_gmt":"2020-05-07T07:44:15","guid":{"rendered":"https:\/\/www.gmass.co\/blog\/?p=4327"},"modified":"2026-01-17T01:31:03","modified_gmt":"2026-01-17T01:31:03","slug":"smtp-services-review","status":"publish","type":"post","link":"https:\/\/www.gmass.co\/blog\/smtp-services-review\/","title":{"rendered":"A technical review of popular SMTP Services [2026]"},"content":{"rendered":"

Over the 20 years I’ve been building email marketing software, I’ve used or come across almost every SMTP service on the market. While plenty of articles exist comparing the pricing and other facets of SMTP relay services, none offer an in-depth technical comparison, which is why I’ve done a deep dive to give you my expert insights below. It’s high time someone that is a technical expert in the SMTP protocol reviewed the popular commercial SMTP services.<\/p>\n

If you Google “SMTP service,” you’ll find a plethora of companies offering services, and in this article, we’ll examine the technical aspects of the most popular ones. This review does not take into account pricing, simply because you can easily look that up on each provider’s website.<\/p>\n

The technical aspects I’ll examine are:<\/p>\n

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  1. The sign-up process:<\/strong> Can you sign up and start sending right away? Is there a human-review required before you’re allowed to start sending?<\/li>\n
  2. Sending limits:<\/strong> Can you send a small 1,000-person campaign right away, or does the service throttle you until you’re a trusted user? (I’ll actually send 1,000 emails to my own users through the service and see what happens.)<\/li>\n
  3. Pass-through or not?<\/strong> Is the SMTP service a pass-through SMTP service that sends your email along just as it was relayed to it, or does it alter the MAIL-FROM headers and add any other information to the headers or body before it transmits the email to the recipient’s MTA? Some services force their own open-tracking pixel into the emails.<\/li>\n
  4. Out-of-the-box SPF and DKIM setup <\/strong>and which domains are used for SPF and DKIM compliance. If it’s a pass-through service, this won’t apply, because it’s impossible to have a pass-through service that is out-of-the-box SPF and DKIM compliant. If it’s not a pass-through service, does it make sure your emails are SPF and DKIM complaint off the bat by using the service’s own domain rather than yours?<\/li>\n
  5. “From” line flexibility:<\/strong> Will it let you send “from” an address @gmail.com?<\/li>\n
  6. Custom DKIM:<\/strong> Is it possible to DKIM-sign the emails myself with my own domain? Is it possible to control the MAIL-FROM header used?<\/li>\n
  7. Shared vs. Dedicated IPs:<\/strong> Will they let you send high-volume email through their shared IPs, or do they aggressively push you towards a dedicated IP?<\/li>\n
  8. The User Interface: <\/strong>For any SMTP service, the SMTP relay system is the most important consideration, but it’s nice when the web UI for reporting and lookups is well designed. Is it slick and modern, or does it look like it was built in the 90s? Most importantly, does it provide three key features: a) the ability to search by recipient and by Subject line, b) the ability to see the exact SMTP conversation with the recipient, and c) the ability to export your data? Does it do thoughtful things like, when it provides DNS records for you to add, give you a “copy to clipboard” widget so you don’t have to drag your cursor over a super long DKIM public key, hoping you get every necessary character but not any extra ones?<\/li>\n
  9. Shared IP reputation:<\/strong> What’s the reputation of their shared IP pool?<\/li>\n
  10. Support competency:<\/strong> Is their front line support personnel competent? Sometimes I find that people performing technical support for email service providers are grossly incompetent, answering basic questions about how email works, entirely wrong. An example awaits you below.<\/li>\n
  11. GMass fit:<\/strong> Finally, we’ll conclude whether the SMTP service is suitable for use with GMass.<\/li>\n<\/ol>\n

    First, what is a “pass-through SMTP service?”<\/h3>\n

    Only an SMTP geek would care about this, and if you Google that term, you won’t find anything because I invented it just now. When you relay an email, and your “from” address is ajaygoel999@gmail.com, and then the SMTP service relays the email to the @gmail.com server, does the MAIL-FROM remain “ajaygoel999@gmail.com”, or does it alter the MAIL-FROM to be “bounce friendly,” so that it becomes ajaygoel999@asdfkljasdf-long-identifier.sendgrid.net? A pass-through SMTP server sets the MAIL-FROM to exactly what the “from” address is when you relayed the email. A non-pass-through SMTP server alters it, based on its desire to control bounces for you, or based on your own customization of the service inside the settings.<\/p>\n

    Why do these things matter?<\/h3>\n

    Depending on what you’re using an STMP service for, the attributes that matter to YOU might differ from the attributes that matter to me. For example, if all you care about is getting your website to send a confirmation email after someone fills out a form, you probably don’t care about the out-of-the-box DKIM setup or whether you’re initially throttled below your paid plan’s sending limits (unless you’re getting thousands of form submissions\/day \u2014 then you might care). But if you’re like me, and you’re using an SMTP relay to service thousands of users, some of whom are sending broadcast email and some of whom are sending transactional email, and you want the process of onboarding the users onto the SMTP relay as seamless as possible, then factors like these matter, along with the process (or hopefully lack thereof) of verifying a new “from” address.<\/p>\n

    What do I look for in the ideal SMTP service?<\/h3>\n

    Sadly, no SMTP service, not even the 600-pound gorilla known as SendGrid, meets all of my strict criteria for perfection. Here they are:<\/p>\n