Hi together,
our customers have big problems sending bulk E-Mails to their signed
members who's domains use a ...protection.outlook.com MX.
After a few hundred mails to different addresses who's domains use a
protection.outlook.com MX, the receiving servers respond with "...451
4.7.500 Server busy. Please try again later..."
So our idea was to configure some additional custom smtp-transports
which could then be chosen by filtering by the MX to be used for the
receiving addresse's domains.
So I configured the three smtp-transports slowersmtp, slowsmtp and
veryslowersmtp in main.cf and master.cf.
Then I thought I could use...
smtpd_recipient_restrictions =
check_recipient_mx_access pcre:/etc/postfix/mx_transport_filter
And in mx_transport_filter I could filter by outlook.com and all other
to choose the assigned smtp-transport.
But when I test it with...
sendmail -t < ./testmail.txt
... a few times fast one after another and use tail -f on the maillog
file, I must watch the e-mails all delivered directly one after another
without the delay of 60sec, configured for the veryslowsmtp.
I must be having some missconfiguration or did not really understand how
it works or should work.
In my desperation I also tried to search the web (all major search
engines) or even ask Perplexity, ChatGTP, Gemini or CoPilot, but they
all gave me all kinds of answers.
I tried a lot of them but none worked.
Could please someone help me solve this problem and tell me what I am
doing wrong or what should be changed?
Here are the relevant parts of my configuration....
------------------------------------------------------------
# postconf | grep smtp_host_lookup
smtp_host_lookup = dns
# postconf | grep disable_dns_lookups
disable_dns_lookups = no
main.cf
------------------------------------------------------------
...
smtpd_recipient_restrictions =
check_recipient_mx_access pcre:/etc/postfix/mx_transport_filter
slowersmtp_initial_destination_concurrency = 2
slowersmtp_destination_concurrency_limit = 1
slowersmtp_destination_recipient_limit = 2
slowersmtp_destination_rate_delay = 2s
slowersmtp_destination_concurrency_failed_cohort_limit = 10
slowsmtp_initial_destination_concurrency = 2
slowsmtp_destination_concurrency_limit = 1
slowsmtp_destination_recipient_limit = 2
slowsmtp_destination_rate_delay = 10s
slowsmtp_destination_concurrency_failed_cohort_limit = 20
veryslowsmtp_initial_destination_concurrency = 2
veryslowsmtp_destination_concurrency_limit = 1
veryslowsmtp_destination_recipient_limit = 2
veryslowsmtp_destination_rate_delay = 60s
veryslowsmtp_destination_concurrency_failed_cohort_limit = 10
...
------------------------------------------------------------
master.cf
------------------------------------------------------------
...
slowersmtp unix - - n - - smtp
slowsmtp unix - - n - - smtp
veryslowsmtp unix - - n - - smtp
------------------------------------------------------------
/etc/postfix/mx_transport_filter:
------------------------------------------------------------
/\.outlook\.com$/ veryslowsmtp
/.*/ slowersmt
------------------------------------------------------------
Many thanks in advance,
regards Henri
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