What is the best topology for providing a second server as a lower priority mailbox server that can continue to accept mail when the primary server is down for any reason (and re-sync when the primary comes back up)? Do we need to go to a 3-server topology with an edge server to route mail to multiple client mailbox servers or use something like SCR to mirror the primary? Or something else?

HI After Exchange 2007 SP2Exchange Server support Online moveso the mailbox could still receive email when moving the mailbox When finish movingjust point the old MX Record to Exchange 2013 Server. Hope that helps cheers If you have any feedback on our support, please click here Zi Feng TechNet Community Support

To get server redundancy in Exchange 2013 with the lowest number of servers you need two. Both server should be multirole servers with all Exchange roles installed. But for this to work you also need a 3rd party load balancer in front of the Exchange servers. You then implement DAG on the two servers to replicate the Mailbox Databases. In short, that is the smallest topology with redundancy that you can implement. Do also note that you do not need a level 7 load balancer with Exchange 2013. In many cases a cheaper level 4 load balancer is enough. Regarding the mail flow you would like to have some sort of spam-filter in front of the two Exchange servers. Edge is one option, but to ensure mail flow you should have two of those as well. Forefront online protection for Exchange (http://www.microsoft.com/exchange/en-us/forefront-online-protection-for-exchange.aspx) is Another option. If you use FOPE then you wont need the local spam-filter and only the two Exchange servers and a load balancer. Hope this helps, let us know if you have any other questions!Martin Sundstrm | Microsoft Certified Trainer | MCITP: Enterprise Messaging Administrator 2007/2010 | http://msundis.wordpress.com

That looks like a good solution but I'll need to figure out if my spam filter gateway can load balance delivery to 2 servers. I'll research and follow up next Monday. Thanks for your input!