I have the same problem, but I'm using Qmail:
When I send a mail from webmail: "smtp.helo=webmail.domain.com"
When I send from Mailclient: "smtp.helo=domain"
I want to configure it, so it sends: "smtp.helo=mail.domain.com" (as specified in MX-record, Plesk Server Settings and rDNS-Record)...