On 20 Apr 2021, at 01:09, Sam Varshavchik <mrsam(_at_)courier-mta(_dot_)com>
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.
laura
[1] In at least one case the decision maker passed away a few years ago.
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