A PHP scri_pt on a Linux (Redhat) web server that I maintain sends email for various types of notifications: new user registrations, replies to forum posts, etc.

Most of these notifications are received, but some are reported as not being received, and I'm trying to figure out why.

I've looked at the sendmail log (/etc/maillog), and can't find any errors. I don't have access to the logs on the receiving side.

Sendmail has a zillion configuration parameters. most of which I don't understand.

Is there a way of getting sendmail to produce a more detailed log, maybe showing the low-level "HELO" type messages?

Not sure about an answer to the direct question- but many errors in this sort of situation will return error when calling your mail function from PHP.

I would recommend checking return values, and logging the result, this is quite likely to lead you to the error quicker.

Please define "reported as not being received" .... based on your next comment, one must assume you are talking about 'user complaints' .. but not sure

Is this server actually handling the e-maiil transfer end-to-end or is there yet another server in the mix? The point being that there is no 'error' recoded on this server because the outgoing was handed off to your upstream with no errors generated, but the upstream server ran into a delivery issue and added that 'problem situation' to its own logs, but not seeing the 'need' to pass that data back to your server.

First reference I stumbled on in a rather simple Google search //sendmail +log// was Configuring and Reading the sendmail Log .. actually sliding down a bit to the section titled Setting Log Levels .... looks like you're wanting at least Level 12, but having to guess/note that this is going to end up with absolutely huge log files.

Also having to note that you may not have looked at any issues involved in the transactions between your PHP scri_pt and the Sendmail application itself .... does the PHP scri_pt include any error handling code, log files, etc.?