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Editorial Team
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Asked: 2026-06-06T02:50:15+00:00

I am currently developing an email server in C, and the end goal is

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I am currently developing an email server in C, and the end goal is to be able to send millions of emails to millions of people every day. Many organizations have email lists with large numbers of users that they email every week/month/etc.

The big question: how can I prevent the server and the emails from being marked as a spam? All of the SPAM-prevention stuff I’ve seen so far deals mostly with poor configurations, or at least does not require large numbers of emails to be send every hour. I have yet to see anything that addresses the scope of millions-of-emails-per-hour.

Here are some assumptions you can make:

  • EVERY single email sent is legitimate
  • all SPF records and MX records are accurate, up-to-date, and valid
  • all other common SPAM-prevention tactics are being used (reverse DNS is good, DKIM is used, return-addresses are valid, etc etc etc)
  • emails are one-to-one (ie, I’m not CC’ing 1000 gmail addresses; I’m sending one email to each address)

Here are some questions to get us moving in the right direction:

  • should I limit the number of emails sent to X emails per minute per domain? If so, how do sites like GMail and MailChimp get around this? note: there are no ISP restrictions; this is only an issue for the receiving mail server…
  • should I limit the number of connections to a domain at a given time? (eg, will Google think I’m a spam agent if I open 10/100/1000 simultaneous connections to gmail servers?)
  • how many bounce-backs (5xx errors on an address) should I accept for automatically removing that email from a subscription list? does this affect a server’s spam rating?
  • is there anything else I should or should not do?

Final note: please remember this is a programming question, NOT a library question – I don’t want to use someone else’s service; we are writing our own for a reason. I’m looking for practical programming advice.

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  1. Editorial Team

    This is not a programming question, but here goes:

    I strongly recommend you join your local mail operators mailing list, as well as “Spam-L” mailing list. Read the archives, and see what issues others are having.

    The short answer is that destination servers can, and do, use all sorts of methods to try to prevent spam. THere are many things you will need to be aware of in order to have good deliverability, and those things change all the time.

    • First and most important, remember:

      Free speech also includes free listening.
      Nobody has to accept or transmit your mail.

      Independent operators, businesses and individuals have a perfect right to refuse your mail for any reason or no reason. ISPs are limited only by their contracts with the customer and common-carrier laws, which generally give them broad discretion in what is considered spam and how they block it.

    • Their system, their rules. If you want your messages delivered, you must cooperate with receiving ISPs. This may mean jumping through hoops, or complying with requirements you think are stupid, or pointless.

    • Ensure you are not listed by SpamHaus. Most ISPs small and large use SpamHaus DNSBL service. Presence on one of SpamHaus’ lists asserts their opinion that your mail meets their listing criteria. Because of SpamHaus’ high reputation, most ISPs will simply block all mail you send based on their opinion.

    • Make sure you process unsubscribes.

    • Make sure you process non-delivery reports. You may not want to kill a subscription on the first NDR, as there can be intermittent network or server problems which can result in non-delivery, or even erroneous reports that an address is incorrect. But if you get several over the course of a month or two with no successful deliveries, you should kill the subscription.

    • Join a pay-for reputation service. These may require posting a bond which you may lose if you send Spam. SpamHaus offer one. There are others.

    • Get professional advice from someone like Return-Path. You will have to pay for this also.

    • Monitor. The hoops you have to jump through change all the time. Ensure you are aware of emerging deliverability problems.

    • Join feedback loops. most large ISPs offer feedback programmes where you can get feedback on how users are perceiving your mail, whether they are reporting it as spam, etc.

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