Re: [ietf-smtp] [Emailcore] Proposed ESMTP keyword RCPTLIMIT

Laura Atkins <> Tue, 20 April 2021 09:17 UTC

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Date: Tue, 20 Apr 2021 10:17:09 +0100
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Subject: Re: [ietf-smtp] [Emailcore] Proposed ESMTP keyword RCPTLIMIT
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> On 20 Apr 2021, at 01:09, Sam Varshavchik <> wrote:
> George Schlossnagle writes:
>> The common limits we see in the real world (in order of most common occurrence/impact) are:
>> Messages per connection
>> Recipients per message
>> Simultaneous connections per sending IP (this would be my number one suggested add)
> This is surprising: that messages per connection is more commonly checked than everything else.
> I would expect that a receiving server would prefer reusing the same connection, to send consecutive messages, than have the sender establish a connection, send one message, then tear it down.
> A long time ago that was Qmail's well-known bad rep: its simplistic implementation, how it created a connection for every individual message. So, a dozen messages to the same domain resulted in dozens of concurrent connections, all to send one message and disconnect.
> I see nothing to gain from forcing a sending server to artificially limit itself; how after every N sent messages it has to close its socket, and reconnect again. What is that supposed to accomplish? I don't get it.

It’s a limit that was implemented more than a decade ago by some of the highest volume receivers around. I don’t think we have to understand why they did it - if even the folks who made the decision are still around. [1] We can just say: this is commonly occurring behavior and it makes sense for SMTP to document it and have a way to explain it. 

I don’t think a discussion about why or if it’s sensible will make any difference. The companies who are closing the connections after X number of messages (5, 10 and 20 are all used) are not likely to either defend their technical decisions to the IETF or change their behavior after a decade and at least one whole infrastructure rearchitecture. 

What we should be doing is figuring out how to make these limits more clear to senders. 


[1] In at least one case the decision maker passed away a few years ago. 

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