i have an actual mail server on a live server (linux/php) which i want to use to send mail...but from another server (windows/php) that is on another IP. the windows server is on a local network so the way i'm accessing it is with domain names not available across the internet. however the windows server is in a demilitarized zone.
I had pointed the SMPT and smtp_port settings in php.ini to the right values and used to send mail all the time this way. However recently i started getting the "SMTP server response: 550 Your HELO string is incorrect" error intermittently. I can't figure out why sometimes it works and sometimes it doesn't.
i'm wondering if maybe its sending through the domain names in my URI in the HELO string and mail server tries to look them up and finds they are invalid? but that still doesn't explain why it works somtimes.