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I have a VPS running CentOS 6. When the system tries to send daily newsletter, after some time (e.g. after sending about 2000 emails), I get error "Unable to send mail" and the system memory goes really high. Till this moment, the mails are delivered normally.

The rest symptoms are:

  1. I cannot see anything on /var/log/maillog (File seems not to be written)

  2. All files on /var/spool/mail have 0 bytes size.

  3. From time to time on httpd log I get errors like: /usr/sbin/sendmail: error while loading shared libraries: libc.so.6: cannot open shared object file: Error 23

"Activate mail service on domain" setting in Plesk is deactivated.

Any idea on what's going wrong here?

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Have a look at your mail queue. Use the command mailq. My favourite error aggregation one-liner is mailq | grep " *(" | sort | uniq -c | sort -n. –  Ladadadada Sep 28 '12 at 10:02
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Are you out of disk space? –  David Mackintosh Sep 28 '12 at 18:02
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1 Answer

postfix (version 2.2 up) does rate limiting with directives in /etc/postfix/main.cf:

smtpd_client_message_rate_limit=1000
anvil_rate_time_unit = 60s

which means a client can only send 1000 mails in 60 seconds.

There are also other systems that can limit message sending as addons to postfix (sqlgrey is an example), they usually inject in /etc/postfix/master.cf or main.cf smtpd_client_restrictions (although not limited to those).

However, from your description, I think that is the least of your problems. You seem to have a wrong or broken version/package installed. (wrong libc, wrong permissions, perhaps wrong file paths).

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