Query regarding sending mail using SMTP service on a local computer

Issue:

We have some programmers who use .net and send some emails, they have smtp service installed on their local desktops.

So when they are sending emails through their code, mail is going to production users, even though on the Smtp Server settings, there is no Smart Host configured.

I have checked the Message header and message is getting accepted by our Mail Marshals from the their Desktops.

We configured our environemnt in such a that we are using two separate Mail Marshals for the emails triggered by applications

I have even checked the logs


#Fields: date time c-ip cs-username s-sitename s-computername s-ip s-port cs-method cs-uri-stem cs-uri-query sc-status sc-win32-status sc-bytes cs-bytes time-taken cs-version cs-host cs(User-Agent) cs(Cookie) cs(Referer)
2010-11-10 13:32:24 - OutboundConnectionResponse SMTPSVC1 PSOBHANI-DT - 25 - - 220+hqddemgw01.xxxxx.com+ESMTP+MailMarshal+(v6.7.2.8378)+Ready 0 0 63 0 265 SMTP - - - -
2010-11-10 13:32:24 hqddemgw01.xxxxx.com OutboundConnectionCommand SMTPSVC1 PSOBHANI-DT - 25 EHLO - psobhani-dt.xxxx.com 0 0 4 0 265 SMTP - - - -


And on our mail marshal we have given "Any" to Relay, as it is for internal apps.

But my question is , how smtpsvc is able to send mail from a local pc and how the mail marshal getway is getting it.