I am not familiar enough with Windows 2003 client access licensing but I am pretty sure there is no limit as to how many active directory users you can create as long as your hard drive space permits. However due to M$ CAL, you may need to buy additional licensing to support more users or device to connect simultaneously, as far as I'm aware, the standard version of 2003 only support 5 CALs. I stay away from M$ licensing by moving to Linux, you can run Postfix or free version of Open-Xchange for email server all equivalent to M$ without the licensing fee. However it takes more skill and time to do it in Linux.