There should be no issues as long as everyone has the same Subnet Mask and Gateway which must reside inside the 192.168.12.1 - 192.168.15.254 range. Do you have any other subnets? Is the mail server also located on this subnet? If you have other subnets check to see if you are using RIP as a routing protocol because V1 of RIP is not classfull, and it will only see a /24 regardless, so you would have to use RIPv2. If you only have the one subnet then my thought is that these issues are not related, and co-incidental. Maybe you changed something else at the same time and wrote it off as not being the problem?
When you say a client has to reboot her machine every day, eplain what happens if they don't.
Also be sure that you updated all essential services, like DHCP, DNS, and all of your firewalls/routers/switches/servers to reflect the new subnet range.
