We’re using sendmail on our server, worked better than postfix in bypassing spam filters. We have certain modules on our site that allow mailing upto 8000 users at a go. However, when we trigger the module(mail sent using php mail()), for a sample queue of 2800 recipients, only 300 or so get the mail…
We had updated the Maximum queue size in sendmail to 0 to make the queue size unlimited, earlier default but still the msgs don’t even enter the queue.
We checked /var/logs/maillog and figured only 300 or so were being delivered.
I unfortunately don’t know much about configuring Sendmail, so I can’t help you much there… however, there’s a few things I’d look into –
First, if Sendmail isn’t going to accept an email into it’s queue, you’d likely see an error regarding that in the mail log.
Are you seeing any attempts to send those other messages that are being rejected by Sendmail?
Also, does your ISP have any sort of policy about outgoing email amounts? ISP’s that aren’t expecting that sort of email traffic could potentially begin blocking outgoing email attempts if they think it may be spam.
Lastly – is it an option to configure your mailing software to send the email in smaller batches? If you attempted to send, say, 200 messages at a time, every 10 minutes, that may work a little smoother.