Hi all,

I have a 2820 with both the ADSL and Ethernet WAN ports connected.

I want to route our SMTP traffic so that it ALWAYS goes along WAN1

any ideas on how to do this?

Fairly easy to do, go to WAN - Load balance policy, on there click on of the Index / links that isn't used. If you've got the "auto failover" tickbox then you're on the right page, set it up like so:
protocol: any
WAN: WAN1 - leave auto failover ticked (or unticked if SMTP won't work through WAN2 without reconfiguring)
source / destination IP - ignore
destination port start+end: 25
OK that and it should put any new SMTP traffic over WAN1