On 2/5/2019 11:31 AM, Paul Smith wrote:
On 05/02/2019 16:11, Hector Santos wrote:
Which smtp servers and client follow or honor Greylist SMTP retry
hint for rescheduling retries?
Well, I thought our server indicated the retry time, but it appears
that we were using a format from a previous discussion about it, not
the proper draft, so we'll fix the return format to match the current
draft.
The client doesn't use the hint though.
(To be honest, I thought the idea had disappeared, so we returned the
retry time because that was a trivial change, but actually using the
hint is a bit more complicated so we were waiting to see if it would
be useful).
As far as minimizing the impact of greylisting delays, it has proven
to be very useful over the years.
Server side:
The default has been set to 55 seconds. My client will honor it and
works as expected among similar setups. Many other clients will try
immediate within the 55 secs and of course, they are blocked.
Yesterday, I was exploring signing up with InstaCart and I was waiting
for their (confirmation/acknowledgement) email. It got in at the 3rd
attempts, 40, then 90 seconds apart. Hence why I posted this to see
if modern services are learning from the greylisting delays.
Client Side:
Yes, you have to adjust your client retry queuing/timing/schedule
logic. The SMTP client code would check for any possible server 4yz
"retry hints" and extracts/returns the recommended time/delta. The
optional delta is then passed your scheduler and used to adjust/set
the next retry. It does work very work to minimize the retries to 2
with the minimum time.
If it helps, I have this C code (port as required for *nix) with the
functions used to extract the time from a response string checking the
learned common strings seen:
// smtpgrey-example.cpp
#include <stdio.h>
#include <windows.h>
//-----------------------------------------------------------
// int GetRetrySecs(const char *psz)
//
// return # of seconds from formats
//
// [DD-]HH:MM:SS formal syntax
// # seconds
// # minutes
// # hours
//
//-----------------------------------------------------------
int GetRetrySecs(const char *psz)
{
char *pOut = NULL;
int days = 0;
int secs = strtol(psz,&pOut,10);
// check for formal syntax [DD-]HH:MM:SS
if (*pOut == '-') {
days = secs;
secs = strtol(pOut+1,&pOut,10);
}
if (*pOut == ':') {
secs = days*24*3600+secs*3600+strtol(pOut+1,&pOut,10)*60;
if (*pOut == ':') secs += atoi(pOut+1);
return secs;
}
// check for informal syntax
if (secs) {
if (*pOut == ' ') pOut++;
switch (*pOut) {
case 's': break;
case 'm': secs=secs*60; break;
case 'h': secs=secs*3600; break;
}
}
return secs;
}
//-----------------------------------------------------------
// int ExtractRetryHint(const char *psz)
//
// For given SMTP 4yz response, return any known retry time
// hint in seconds. Return 0 for no time hint or non 4yz
// response. The routine checks for the formal syntax and
// for non-formal existing syntax:
//
// retry=[DD-]HH:MM:SS proposed draft std
// greylisted for # seconds|minutes|hours common, non-std
// try again in # seconds|minutes|hours common, non-std
// please come back in HH:MM::SS common, non-std
//-----------------------------------------------------------
int ExtractRetryHint(const char *psz)
{
int nsecs = 0;
if (!psz || !psz[0] || ((atoi(psz)/100*100) != 400)) {
return nsecs;
}
const char *tags[] = {
"retry=",
"greylisted for ",
"try again in ",
"please come back in ",
NULL};
const char **p = tags;
char *lpsz = _strlwr(_strdup(psz));
while (*p) {
char *pos = strstr(lpsz,*p);
if (pos) {
nsecs = GetRetrySecs(pos+strlen(*p));
break;
}
p++;
}
free(lpsz);
return nsecs;
}
void test(const char *sz)
{
int secs = ExtractRetryHint(sz);
printf("secs: %5d | %s\n",secs, sz);
}
void main(char argc, char *argv[])
{
test("421 This server implements greylisting, please try again in
90 seconds");
test("450 4.7.1 <RCPT>: Recipient address rejected: Greylisted
for 5 minutes");
test("450 4.7.1 <RCPT>: Recipient address rejected: Greylisted
for 120 seconds");
test("451 4.7.1 Greylisting in action, please come back in
00:30:00");
test("451 4.7.1 Greylisting in action, PLEASE COME BACK in
01:05:20");
test("451 Greylisted for 55 seconds");
test("451 Greylisted, please try again in 167 seconds");
test("451 Greylisting enabled, try again in 22 minutes");
test("451 Greylisting. Try again later.");
test("451 Greylisting. Try again later. retry=5 mins");
test("471 Greylisting. Try again later. retry=300 secs");
test("471 Greylisting. Try again later. retry=2 hours");
test("471 Greylisting. Try again later. retry=2h");
test("471 Greylisting. Try again later. retry=120");
test("471 Greylisting. Try again later. retry=01:01:02");
test("471 retry=01:01:02 Greylisting. Try again later.");
test("471 Greylist: RETRY=1M Try again later.");
test("550 Permanent Reject");
}
--
HLS
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