We're a medium sized regional MSO/broadband provider with 200k+ mailboxes, strongly considering enabling SMTP authentication on our customer-facing SMTP mail servers.
We're relying exclusively on SMTP AUTH for SMTP relaying. The single biggest issue is that it requires ongoing user education. After a few weeks people forget what they did do get rid of the "Relaying denied" error message. It doesn't help that SMTP AUTH is not an option the "New Account Wizard" of Outlook and Outlook Express asks for. It has to be setup manually after. For "our" mail users it has been well received, ignoring the support calls. A big advantage is that roaming user no longer have to worry about who ip space they're on. That may change as networks install SMTP blocks. Adi