> 1. Try adding "unknown_address_reject_code = 553" in main.cf. This is a
> wise choice in any case.
> 2. Use a better mail client which handles multiple recipient mail more
> robustly.

Thanks for the pointers!

I added the 553 reject code, but unfortunately it does not seem to help.

My client is msmtp and the SMTP session looks like this:

cat mail.txt | msmtp -a default -X mail.log --debug -t

<-- 220 REDACTED ESMTP Postfix (Ubuntu)
--> EHLO localhost
<-- 250-REDACTED
<-- 250-PIPELINING
<-- 250-SIZE 10240000
<-- 250-VRFY
<-- 250-ETRN
<-- 250-STARTTLS
<-- 250-ENHANCEDSTATUSCODES
<-- 250-8BITMIME
<-- 250 DSN
--> MAIL FROM:<REDACTED>
--> RCPT TO:<o...@linux.com>
--> RCPT TO:<REDACTED>
--> DATA
<-- 250 2.1.0 Ok
<-- 250 2.1.5 Ok
<-- 553 5.1.2 <REDACTED>: Recipient address rejected: Domain not found
msmtp: recipient address REDACTED not accepted by the server
msmtp: server message: 553 5.1.2 <REDACTED>: Recipient address
rejected: Domain not found
msmtp: could not send mail (account default from /etc/msmtprc)

So if the blame is on the client, what replacements for msmtp do you recommend?

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