Excursus 451 452 Retry Strategies

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dilyan.palauzov
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Excursus 451 452 Retry Strategies

Post by dilyan.palauzov » 2019-07-29 20:07

Imagine, a mail envolope contains many recipient, The server accepts the first recipients and rejects the last recipients, meaning “Too many recipients in this transaction”.

RFC 821 specifies the reply code 452 as “Insufficient storage”, which RFC 5821 amends, by stating that 452 can mean also too many recipients in this transaction.

RFC 3463 defines enhanced status code 4.5.3 stating “Too many recipients”. RFC 5248 attaches the ESC 4.5.3 to reply code 451, stating that changing this binding requires a specification, and there is no such specificaton. The latter means, that 452 4.5.3 is not valid. Postfix and sendmail send “452 4.5.3”, exim sends on this occasion 452 without ESC.

How can hMailServer be tweaked to retry immediately:
• Does hMailServer interpret 4.5.3 anyhow special?
• Does it handle 451 and 452 differently by default?
• If a site publishes many MX records pointing to the same IP address, will hMailServer do a lot of tries (reducing the amount of pending recipients in each iteration) in shortes time?
• If a site published one MX records pointing to many different IPv6 addresses, will hMailServer do a lot of retries to deliver the same message in shortest time?

I guess, that a site publishing many MX records pointing to many IP addresses is not an additional option to increase the retry rate.

dilyan.palauzov
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Re: Excursus 451 452 Retry Strategies

Post by dilyan.palauzov » 2019-07-29 20:18

SpamAssassin recomends inserting fake (lowest and highest) MX records to reduce Spam. E.g. MX aegee.org resolves to 90 mxf-2.aegee.org. / 10 mail.aegee.org. / 1 mxf-1.aegee.org. and on mxf-1,2.aegee.org there is no SMTP server.

For political reasons I cannot send the link to the recommendation from SpamAssassin in this forum. The URL is https + cwiki.apache.org + slash + confluence/display/spamassassin/OtherTricks

What is the impact of fake records on the retry strategy?

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mattg
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Re: Excursus 451 452 Retry Strategies

Post by mattg » 2019-07-30 00:50

hMailserver has settings under SMTP >> Delivery of email

Number of retries and minutes between every retry
Every 45X code gets the same response, hMailserver retries as per those settings.
Once retries is exceeded the message is bounced back to the sender as a non delivery.

There is also a setting under SMTP >> Advanced which asks 'Maximum number of recipients in batch'
This tells hmailserver to limit the number of recipients for each recipient mail server. You can set this 1 if you like and then you will never have your issue

(Honestly for real world use 10, 15 or even 100 is probably fine. I have mine set to 100, but I don't send newsletters and such)
Just 'cause I link to a page and say little else doesn't mean I am not being nice.
https://www.hmailserver.com/documentation

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