PHP unable to mail() in chroot though I can use sendmail inside chroot
I know this is quite an old question, but I found this as one of the hits while trying to solve the same issue and so in case anyone hits this in future there's another thing to check. In my case I was missing /bin/sh from the chroot environment.
php passes sendmail_path into popen. popen passes the command to /bin/sh and so if your chroot environment is missing /bin/sh you seem to get a failure with nothing logged