I'm trying to diagnose a problem with certain email addresses possibly being blocked on my server. I'm running PHP 5.3 on CentOS 5.7. The php.ini file lists a sendmail_path of /usr/sbin/sendmail -t -i, which when run in CLI hangs there. I've noticed that qmail is installed on my server, too, but I don't know if PHP is using it or not.How do I find out which MTAs (i.e. sendmail, qmail, etc) PHP is using?
What Mail Transfer Agents is PHP using?
centos;email;php;mail transport agent
TL;DR: PHP doesn't care about what MTA you're using.Longer explanation: this goes way back almost as far as the POSIX standards themselves, but every properly written MTA will provide a binary named sendmail that will behave exactly as the official sendmail program would be expected to behave.As a result, every unix program or daemon that, for one reason or other, finds itself needing to email someone, knows they can just call /usr/sbin/sendmail with known options, and be confident that whatever MTA was installed will know what to do with the message from there on in. As such, unless you use a specific SMTP PHP module and explicitly to use different mail settings (generally, a remote server/port with or without TLS and/or authentication), it will just call /usr/sbin/sendmail and let the underlying distribution worry about what happens next. If your mail isn't arriving, I recommend you check the error logs of the MTA (usually in /var/log/mail.* but depends on your distribution and MTA) for answers.