Google doesn't work well with emails handed over SMTP that have multiple domains that are hosted with them. Attempts to send quite valid messages
like this will get the status:

451-4.3.0 Multiple destination domains per transaction is unsupported.

How have you resolved this, or have you even bothered? Sendmail retries
the failed message (apparently during the same session) and succeeds
once the message has been pared down to a single domain.

--
/* * * Otto J. Makela <om@iki.fi> * * * * * * * * * */
/* Phone: +358 40 765 5772, ICBM: N 60 10' E 24 55' */
/* Mail: Mechelininkatu 26 B 27, FI-00100 Helsinki */
/* * * Computers Rule 01001111 01001011 * * * * * * */

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Am 14.09.2023 schrieb om@iki.fi (Otto J. Makela):

Google doesn't work well with emails handed over SMTP that have
multiple domains that are hosted with them. Attempts to send quite
valid messages like this will get the status:

451-4.3.0 Multiple destination domains per transaction is unsupported.

I don't know why Google is unable to support common standards like
multiple recipients in one mail.

How have you resolved this, or have you even bothered? Sendmail
retries the failed message (apparently during the same session) and
succeeds once the message has been pared down to a single domain.

https://groups.google.com/g/comp.mail.sendmail/c/QCdqRFlYtOs
Although this is a limit for all outgoing connections according to what
I read.

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Claus helped out with this a few months ago, search the group for "bestmx"

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