Office365 restricting new email server using sendmail.

[markr...@gmail.com]

Hi - I have a brand new email server with sendmail that I'm testing, but it's getting error log messages like "Deferred: 452 4.5.3 Too many recipients (AS780090)" from Office365 servers, even when very few recipients are specified in email. The only thing I've found on the web that perhaps applies is below. Although the people weren't using sendmail.. Could this be my problem? I wonder if I've accidentally turned TLS off. Is there a way to check?

Or might this be a different problem? If no one knows, is there anywhere else I can ask my question? Thanks! - Mark

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The underlying issue appears to be a default security "feature" associated with Office365 that we not extant in Microsoft Exchange.

Apparently mailservers that can't use TLS transport for handshake to Office 365 are limited to a Microsoft default of a single recipient per message to be delivered. That value could be overridden on the *destination* Office365 server,

[Marco Moock]

Am 19.06.2023 um 02:47:29 Uhr schrieb markr...@gmail.com:

> Hi - I have a brand new email server with sendmail that I'm testing,
> but it's getting error log messages like "Deferred: 452 4.5.3 Too
> many recipients (AS780090)" from Office365 servers, even when very
> few recipients are specified in email.

It is possible to reduce the amount of recipients in sendmail.
https://www.oreilly.com/library/view/sendmail-4th-edition/9780596510299/ch20s05s14.html

[Claus Aßmann]

markr...@gmail.com wrote:

> it's getting error log messages like "Deferred: 452 4.5.3 Too many
> recipients (AS780090)" from Office365 servers, even when very few
> recipients are specified in email. The only thing I've found on the web

What are "very few"?

Also see cf/README
SMTP_MAILER_MAXRCPTS [undefined] If defined, the maximum number of
recipients to deliver in a single envelope for the
smtp, smtp8, esmtp, or dsmtp mailers.

> that perhaps applies is below. Although the people weren't using
> sendmail.. Could this be my problem? I wonder if I've accidentally
> turned TLS off. Is there a way to check?

Look for STARTTLS=client in the log and check the relay= value
for the name of those "Office365 servers".

Also check whether your sendmail version supports TLS:
sendmail -bt -d0.13 </dev/null 2>&1 |fgrep STARTTLS

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[markr...@gmail.com]

From further research, it appears that Office 365 servers are picky with new servers, and they require them being run long enough to gain a good reputation. A lot of people complain about this. Email will often be deferred.

So I switched ip addresses for my new mal server. with another email server I have, that only sends out a small amount of email every often.

Voila. Now my new mail server has no problems with Microsoft servers. No matter how much email is sent out.

Stupid Microsoft! Thanks. - Mark