Background: Customer hosts an on-premise Exchange 2013 server, as well as a web-server that is critical to their operations. They can send and receive email perfectly well from on-site however due to the fact that the https and http ports are forwarded to this other web server the only way they can use email offsite is via IMAP. To complicate things further they are relaying their outgoing email through their isp's mail server due to some issues with spam filtering. This appears to prevent them from sending email out through their email server when offsite we have had to route their outgoing email through a 3rd mail server to allow them to send email from phones, unfortunately this has been causing regular issues.
Current Situation: We are attempting to get a second WAN interface set up on their Sonicwall TZ105 which will then be set up to communicate directly with the exchange server, in an attempt to be able to forward the proper ports and allow them to actually use exchange from outside of the office rather than IMAP. The issue is that their ISP hands out their WAN IP's DHCP via the WAN Interface MAC address, they are ultimately static IP's but not set statically in the router. All of these IP's are in the same /24 subnet which creates a conflict in the Sonicwall where it refuses to obtain the second IP DHCP and even if I attempt to plug in the information statically I get an IP conflict error and it won't take the configuration.
What I need to know is if there is any way I can configure this to allow me to use Exchange from offsite, either through the second WAN IP or some method I've overlooked?