Zen Cart 1.3.8a, smtpauth, Windows 2003, IIS, MailEnable, new install, very few customizations. www.stdtime.com/store.

When some people attempt to order, they get an SMTP error (with no details). The mail server logs show the error below.

RCPT TO:<> 501 Syntax error in parameters or arguments

Evidently, the "to" email address is empty in RCPT TO. But again, this only happens with some people. Most people who order are fine.

All my email options have an email address. Any idea which field is empty?

Regards,
Ray White
719-488-8697 x2