Have recently installed a small linux router running Debian GNU/Linux in front of my mail server, I have forwarded port 25 and 110 to the internal mail server however when trying to start exim4, I retrive the error Failure to bind port 25 to x.x.x.x Does anybody know what I can do to fix my mail system?

Sounds like you have another MTA bound to port 25 on the mail server.

You can run netstat to see what's bound to the port and then you can use ps aux to get the pid of the offending process, then kill that process.

Find the binary for that service and chmod -x /path/to/mta

I can't see that that is the case given the output of netstat
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp6 0 208 ::ffff:192.168.0.:20022 cpc2-wilm1-0-0-cus:1893 ESTABLISHED
tcp6 0 0 ::ffff:192.168.0.:20022 cpc2-wilm1-0-0-cus:3839 ESTABLISHED
tcp6 0 0 ::ffff:192.168.0.:20022 cpc3-wilm1-0-0-cu:63477 ESTABLISHED
Active UNIX domain sockets (w/o servers)
Proto RefCnt Flags Type State I-Node Path
unix 2 [ ] DGRAM 3085 @/org/kernel/udev/ude vd
unix 10 [ ] DGRAM 5326 /dev/log
unix 2 [ ] DGRAM 10354
unix 3 [ ] STREAM CONNECTED 10351
unix 3 [ ] STREAM CONNECTED 10350
unix 2 [ ] DGRAM 10088
unix 3 [ ] STREAM CONNECTED 10085
unix 3 [ ] STREAM CONNECTED 10084
unix 2 [ ] DGRAM 9658
unix 3 [ ] STREAM CONNECTED 9655
unix 3 [ ] STREAM CONNECTED 9654
unix 2 [ ] DGRAM 7411
unix 2 [ ] DGRAM 6126
unix 2 [ ] DGRAM 5568
unix 2 [ ] DGRAM 5361
unix 2 [ ] DGRAM 5341

However, I should probably note that the ip the mail server is trying to bind to is not directly on any of the interfaces of the mail server as it is the ip of the WAN interface of the router