"The odd thing is that a regular email sent from a computer using the address arrives, but the email generated by the script does not, which seems odd and suggests there's something in the script contributing to the issue."
Actually, this suggests that 2 different SMTP servers are being used, the one the script uses to send the email has that address grey listed, but the one your email client connects to does not.
really isn't anything that can be done about it other than setting universal email to use a different SMTP server or contacting the host to have that address removed from the grey list.