We've got a 2820n with the 3.3.4 firmware installed to it.

We are using this with our SDSL servcice plugging into WAN2 of this router. We also have a block of IPs that we have assigned and use these for various services.

We have started noticing that our mail server jumps from one WAN IP address to another and doesn't stick to any one for an extended period of time. This is causing us some issue as it causes some mail to be rejected due to reverse DNS lookup failure.

Does anyone know how I can fix it so that this server only uses one IP address and no other out of our pool of addresses?

Ahh, multinat, I believe the only way to do it is by enabling DMZ mode for that internal ip, that then locks all inbound and outbound traffic to the external ip of your choice.

http://www.draytek.co.uk/support/kb_vigor_multinat.html

But then you'd have to be pretty sure your mailserver was locked up nice and tight.

I had the same problem - I got another line with a single static ip.

To make sure it just uses the one IP, make sure that if you're using WAN IP alias that you dont have "join NAT IP pool" ticked for other addresses than the main one.
If that still doesn't fix it, set up Address Mapping under the NAT menu, which will lock a local IP to the specified WAN IP.

When I had the issue it was with a 2600+ that had no NAT address mapping available - I run a few 2820's now - let us know if Voodle's suggestion works!

Thank you for all the replies
After I posted it I put my Google hat on and did a bit of digging. I found the Multi-NAT discussion in the Draytek vFAQ of all places!

Found the correct menu within the 2820 and found the Multi-NAT was ticked. Unticked all and applied those settings - all sorted!!