> 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?