Without seeing the SMTP error message its hard to say what the issue is. I would suggest adding authentication because most SMTP service provider will require it to relay. Hope that points you in the correct direction.
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I just setup a new Workcentre 6605 printer and it seems that the scan to email only works to certain addresses. For example, it works to send scanned files to my Gmail account but not to my work account which is a server in another country. My service provider says that there is no need for SMTP authentication, and again it indeed works to scan to my gmail account. So the SMTP seems to be working correctly. Where might I look next??