[Zope] mailhost question

I've been trying to use the how-to on "How to create a mail form" to send e-mails from our Zope-based site running on Linux. All goes as advertised except that I get an SMTP error 553: Domain Name Required back when I try it if I use localhost as the smtp host name. I sent mail to myself from the admin account on the hosting server and used the domain name in the from address as the Local Host setting in the MailHost object. I tried changing the SMTP host setting to our corporate server (after getting our admin to give permission to the zope server IP addr.) and the error message changes to SMTP eror 551: none Can anyone point me in the right direction here?