In port forwarding in my previous router I was able to lock down port 25 to a certain WAN ip address range that corresponds to my 'email security provider'.

In the 2830n in port forwarding I can only specify wan1, wan2 and not an external range to allow connections from.

Could I have some pointers as to how to do this? Am assuming I must define a firewall rule somewhere...

Thanks!

have you got a subnet of external addresses?
If you do then you need to create them as alias and then you use open ports and select the alias you want

If how ever you been you want to accept mail from the services provider IPs then you need to do it in the Datafilter
Create a Block all unless further match rule for SMTP and then under it create an Allow rule for the IPs you want to receive mail from for SMTP