http://blogs.msdn.com/andrewarnottms/archive/2007/10/29/what-do-you-think-of-the-new-wcf-store-and-forward-mail-transport.aspx

I can't speak for the mail transport in WCF, because I've not yet used it, however I've used other mail transports with .NET before.  The nice thing about the mail transports is that it is a technology that crosses firewalls easily.  If you have a client outside the firewall, it is almost never a problem for it to e-mail someone inside the firewall.  It is usually easy to get an e-mail account (regardless of the server software involved).

E-mail transports are also useful because Microsoft Message Queue requires a domain controller for some distributed scenarios, but you are sometimes on leased servers that aren't domain controllers.  In those scenarios, e-mail transport provides most of the same benefits as message queues, but without the need for a PDC.  That is huge for some setups.

The other really nice thing about e-mail transports is that given backup MX service (something that only costs around $20/year from companies like no-ip and the like), servers can go offline -- even for a week or more -- and the messages will recover when it comes back up.

The big drawback to e-mail is that it is very high latency.  The messages might get there in 10 seconds.  The messages might get there in an hour.  The messages might get there in a week.  This usually isn't a problem on internal e-mail, but it is a big problem for system domain boundaries. 

Also, you have to watch for filtering as well.  For example, is the remote system allowed to send mail by the SPF?  I am a huge advocate of SPF -- I have it set up on my servers, I enforce it on inbound mail, and you should too.  SPF blocks more than 300 messages a day on battlebazaar.com, and around twenty to thirty a day on battlebazaar.net, and we don't have a ton of users (but battlebazaar.com was used in several discussion lists and newsgroups, and so the spammers came).

Many anti-spam programs will trigger off of message headers, and off of receiving many similar messages.  You have to be certain whatever anti-spam you have server-side is able to handle the messages reliably also.