Why do ISPs limit emails per connection?


A few years ago it was “common knowledge” that if you were sending large amounts of email to an ISP the most polite way to do that, the way that would put the least load on the receiving mailserver, was to open a single SMTP session to the mailserver and then to send all the mail for that ISP down that single connection.
That’s because the receiving mailserver is concerned about two main resources when handling inbound email – the pool of “slots” assigned one per inbound SMTP session, and the bandwidth (network and disk, and related resouces such as memory and CPU) consumed by the inbound mail – and this approach means the sender only uses one slot, and it allows the receiving mailserver to control the bandwidth used simply by accepting data on that one connection at a given rate. It also amortizes all the connection setup costs over multiple emails. It’s a beautiful thing – it just doesn’t get any more efficient than that.
That seems perfect for the receiving ISP – but ISPs don’t encourage bulk senders to do this. Instead many of them have been moving from “one connection, lots of mail through it” to “multiple connections, a few messages through each”. They’re even limiting the number of deliveries permitted over a single connection. Why would that be?
The reason for this is driven by three things. One is that the number of simultaneous inbound SMTP sessions that a mailserver can handle is quite tightly limited by the architecture of most mailservers. Another is that the amount of mail that’s being sent to large ISP mailservers keeps going up and up – so there are sometimes more inbound SMTP sessions asking for access than the mailserver can handle. The third is that ISPs know that there are different categories of email being sent to their users – 1:1 mail from their friends that they want to see as soon as possible, wanted bulk mail that their users want to see when it arrives and spam; lots and lots of spam.
So ISPs want to be able to do things like accept 1:1 mail all the time, while deferring bulk mail and spam to allow them to shed traffic at times of peak load. But they can only make decisions about whether to accept or defer delivery in an efficient way at SMTP connection time – they pick and choose amongst the horde of inbound connection attempts to prioritize some and defer others, letting them keep within the number of inbound sessions that they can handle simultaneously.
But once the ISP lets a bulk mailer connect to deliver their mail, they lose most of the ability to further control that delivery as the sender might send thousands of emails down that connection. (Even if the ISP has the ability to throttle bandwidth – as some do to control obvious spam – that just means that the sender would tie up an expensive inbound delivery slot for longer).
So, in order to allow them to prioritize inbound connections effectively the ISP needs to terminate the session after a few deliveries, and then make that sender start competing with other senders for a connection again.
So ISPs aren’t limiting the number of deliveries per SMTP connection to make things difficult for senders, or because they don’t understand how mail works. They’re doing it because that lets them prioritize wanted email to their users. The same is true when they defer your mail with a 4xx response.
It might be annoying to have to deal with these limits on delivery, but for legitimate bulk mail senders all this throttling and prioritization is a good thing. Your mail may be given less priority than 1:1 mail – but, if you maintain a good reputation, you’re given higher priority than all the spam, higher priority than all the email borne viruses, higher priority than all the junk email, higher priority than the 419 spams. And higher priority than mail from those of your competitors who have a worse reputation than yours.

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  • Another reason is that some senders (both legit and otherwise) will leave connections open for a really long time between messages, which means those sockets can’t be used by other senders unless the ISP disconnects. In effect, it’s a distributed denial of service attack.

  • Hi,
    Does anyone knows what are the inbound limits for Gmail and Hotmail? I mean, what kind of limits do i have to set on my MTA?

  • Hi Laura,
    Thanks for your response. I have already checked the ISP Information page and the info there is not complete… Also, the information that shows is the ISP Outbound Policy and not the Inbound, correct?
    What i mean is that the information that i see in ISP Information Page is the same information that ISP’s announce has their Outbound email sending limits to users that are using their services.
    Im an ESP and what im looking is for information about the ISP Inbound policy (in a simple way, im looking for the information that i have to set up on my MTA in order to meet ISP’s Inbound policy).
    Can you help me on this? The Outbound policy for the ISP’s is the same as the Inbound policy?

  • The information that shows on the page is Inbound policy. As for completeness, a lot of ISPs do not publish the data you’re looking for. You can try looking at postmaster pages for the ISPs you’re interested in. http://postmaster.*/ or http://*/postmaster are two of the more common formats for the postmaster pages.

By steve

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