[BlueOnyx:25548] Re: Error message while sendung to gmail

[Dirk Estenfeld]

Hello Michael,

not only Google. There are even more...

It is (unfortunately) still a 5208R.
so there is only MAILER('procmail') and MAILER('smtp').
But I put your two lines above these two lines and now I'll let the customer send mails to gmail again and we'll see what happens.

Thank you for the tip and best regards
Dirk


blackpoint GmbH – Friedberger Straße 106b – 61118 Bad Vilbel 

 

-----Ursprüngliche Nachricht-----
Von: Blueonyx <blueonyx-bounces at mail.blueonyx.it> Im Auftrag von Michael Stauber
Gesendet: Mittwoch, 27. Juli 2022 21:07
An: blueonyx at mail.blueonyx.it
Betreff: [BlueOnyx:25539] Re: Error message while sendung to gmail

Hi Dirk,


> we have received an error message from a customer that he gets when he 
> specifies several different gmail recipients in one email:
> 
>      ----- Transcript of session follows ----- ... while talking to 
> gmail-smtp-in.l.google.com.:
> 
>  >>> DATA
> 
> <<< 451-4.3.0 Multiple destination domains per transaction is unsupported.
> 
> I assume this has to do with the fact that sendmail connects once to 
> the MX when the domain is the same and then transmits all mails to the 
> same domain and gmail seems to like that not (anymore?).
> 
> Is there any way to change the setting in sendmail or is a solution to 
> switch from sendmail to postfix?


Yeah, this is basically Google being assholes. The proper fix would be for them to behave like a good netizen, but that probably would be too much to ask for.

What's happening here is this:

If you have an email that goes to multiple recipients and more than one of the recipients of that email has a Gmail account?

In that case Google shows you the middle finger, as they don't allow the same email going to more than one recipient in the same transaction.

So what can you do on your end? In sendmail.mc one could insert the lines ...

define(`SMTP_MAILER_MAXRCPTS', `1')dnl
define(`RELAY_MAILER_MAXRCPTS', `1')dnl

... before these ...

MAILER(`local')dnl
MAILER(`smtp')dnl

... and that limits Sendmail to send emails to multiple receivers. From the Sendmail cf/README:

--------------------------------------------------------------------
SMTP_MAILER_MAXRCPTS [undefined] If defined, the maximum number of recipients to deliver in a single connection for the smtp, smtp8, esmtp, or dsmtp mailers.

confMAX_RCPTS_PER_MESSAGE MaxRecipientsPerMessage [infinite] If set, allow no more than the specified number of recipients in an SMTP envelope. Further recipients receive a 452 error code (i.e., they are deferred for the next delivery attempt).
--------------------------------------------------------------------

Side effects: You're crippling your mail-server and are shooting yourself in the foot. This will force your MTA to make one bloody connection for every bloody email it tries to send. If an email has 100 recipients, then in the past that was broken down to one delivery attempt to all (or batched to as few as possible). Now it means 100 individual attempts.

Performance wise this will mean your email performance takes a big hit and delivery gets slowed down. And then you will have clients complaining and reminding you that ...

"When I use my Google email account I *NEVER* have these kind of problems! It's only with *YOUR* server!" \o/

Ain't that cute? But it's even worse:

https://support.google.com/mail/answer/81126?hl=en

https://twitter.com/erdgeist/status/1532714091269201920

https://www.spamresource.com/2020/11/honestly-dont-send-to-gmail-over-ipv6.html

Read especially the last link.

Google is using reputation based filtering. If you set up a new server which happens to use IPv4 and IPv6 and try to send email to Google? 
You're basically doomed. They won't accept it. Even if you don't use
IPv6 and the mail-server is new? You don't have a reputation with them yet, so there is a high chance you're not getting through. And even if you do: Any kind of moderate initial volume of emails from you to Gmail will directly raise red flags on their end, tainting your reputation.

Bottom line: They're not playing nice and are piss poor neighbors that try to force you to play by their arbitrary rules. And complying with their rules will hurt you.

Will switching to Postfix help? It doesn't solve the reputation issue with Gmail or other fuckery, but Postfix queues a little different, so it *might* help a little with that particular problem.

--
With best regards

Michael Stauber
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