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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:
Here are some questions to get us moving in the right direction:
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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Spammers buy email addresses from special providers in bulk to add them to their mailing lists. If you've noted a sudden increase in the number of spam emails landing in your account, there's a high chance that your address was part of a list recently sold to one or more scammers.
Spammers use spambots to crawl the internet looking for email addresses that are used to create email distribution lists. The lists are used to send junk email to multiple email addresses -- usually hundreds of thousands -- at one time.
If you send 10,000 emails at once and there's a high bounce rate, you'll get blacklisted when the rate goes above 10%. Sending fewer emails as you warm up the IPs will help prevent that situation. Remember, more is not always better when it comes to sales leads.
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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Ben had some good practical advice, but for others with this problem, here is what I have discovered in the past month:
Email is all about REPUTATION. You will never be able to throw together a server, ip, and/or domain name and expect to be able to send out millions upon millions of emails.
On Stack Overflow, we have a rating system (up and downvotes) to estimate the value/trust that person has with the SO community. But it takes time and effort to get points. It's the same with email - you have to start sending out small amounts of email that people actually open up and read (and would never mark as spam), and then slowly send out more and more every month until you reach the goal of millions and millions of emails.
Everytime someone "downvotes" - marks the email as spam, flags the domain, flags the ip address, deletes the email without reading it, etc - you get a hit against your reputation. You need to be continually monitoring and putting effort and best-practices into your reputation if you want to gain good standing with people.
So start small, expand in a stable and steady manner, and always keep a watchful eye out for abuse, misuses, good and bad feedback, or anything else that might affect your reputation.
It's not only possible, but very practical; you just need to give it time and effort.
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