SMTP relay

Can someone explain to me the reason I would need an SMTP relay?

I have an exchange 2003 server on site.
Users access email within the "network" using Outlook 2010.
and outside the office via webmail,, and Exchange on iPhone, Blackberry's, mail for exchange on Nokia's and Exchange on HTC's.

Our website sends emails out from one of our email addresses using SMTP.

My question is, do we need an SMTP relay, what does it do, and why do we need it?
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Minoru7Connect With a Mentor Commented:
SMTP relays are used to route mail that is coming from an internal server to external email addresses without first authenticating with Exchange.  The main reasons for needing something like this would be to send alerts or other such things to an external email address, or sometimes you may need it to send internally if an application is unable to authenticate with Exchange.  As long as any clients within your company are able to authenticate (send username/password) to Exchange, then a relay is not necessary.  Keep in mind that you never want to setup an open relay.  If a relay is necessary, make sure to lock it down in some way, even if only by the IP address of the specific client that will be using it.
antoniokingAuthor Commented:
Thanks for the quick reply.
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