Here's the situation: I've got a Redhat Linux server with PHP 5.2 and standard sendmail that we're using to send email to a list of 10,000 email addresses. We have a fairly simple script that uses PHPMailer and connects over SMTP to the listening sendmail daemon.

Whenever this script runs, it'll eventually get to the point where it will just start running dirt slow - it'll start off fast, but then slow down to only sending 1 email every few minutes. When I restart the script, it runs fast again for a bit.

Over on the sendmail server, I've added a few settings in the script to attempt to fix the throttling issue:

define(`confBAD_RCPT_THROTTLE', `0')dnl
define(`confCONNECTION_RATE_THROTTLE', `0')dnl

So far, I'm not sure the best way to fix the problem - or to even troubleshoot it. The /var/log/maillog file doesn't really have any useful information in it.

We've got this sendmail instance behind a firewall, so we don't need to worry about security... just open it up and let the mail flow!

2 Answers 2


You shouldn't be connecting to the remote SMTP by script, but deliver to the localhost using sendmail (or better, postfix). Sendmail should take care of delivering the messages. If you need to make sure all messages go out through a specific gateway machine, configure sendmail with the correct smarthost.

The reason you want to do this is that sending mail is much more complex then you might initially think, and sendmail (or better, postfix) is built to handle all of the gory details like queue management.

As for what's happening on the mail server itself, have you checked the load and other factors on that server? It might be overloaded after you've sent so many messages to it, and is legitimately slow because of it. Check things like 'top', 'netstat -n', etc...

  • When I use sendmail directly, the email messages that go out have a "from" in the headers, which is causing issues with spam filters. Is there a way around that? Feb 22, 2010 at 21:02
  • frequently shows up in email headers, as it's often used to resubmit messages to "myself" to perform operations like spam filtering, antivirus scanning, etc... I think if a spam filter is complaining about this, the filter should probably be adjusted. Feb 23, 2010 at 23:55

You may want to try increasing the log verbosity on sendmail. That may give you some more clues.

You may need to try to determine if the latency is coming from the PHP side or from the sendmail side.

When it get's slow, run a netstat and see what ports are open.

Double check DNS. Make sure your server can quickly resolve domain names. Often I run a caching DNS server locally when sending large volumes of email.

I've not used PHPMailer in this fashion but I've been able to just call /usr/sbin/sendmail via a php script as see 600+ messages/minute on modest hardware.

  • Forgot to add, make sure you are using multiple queues in sendmail, check your timeouts and other SMTP side tuning. The server side tuning on sendmail can make a huge difference in your send rate, especially if you are sending a lot of emails to the same domain. Feb 22, 2010 at 20:21
  • Would that make the script hang when trying to connect to SMTP? Feb 22, 2010 at 20:25

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