MS Exchange server and other mail servers represent just a sub-set of the services and applications included in Starkit.
You can find here more details how Starkit compares with Microsoft Exchange.
1. MS Exchange is not complete solution, apart from the mail server software, you need to buy separate software or appliances like: windows operating system, separate anti-spam and anti-virus, mail client software (MS office), email archiving. This will sky rocket your cost.
2. Set up a mail server requires extensive technical skills to do it properly and securely. Need to pay for labour and also need to have a specialized trained admin to maintain your system
3. MS Exchange will still be missing components such as cloud storage, advance contact management, sharing and collboration features, disk encryption, built-in backup, high performance webmail and ultra fast company wide search and email archiving.
Summarizing, Starkit includes more components, is more secure and is a zero maintenance solution than MS Exchange or any other mail server. Starkit comes at low cost, does not extra software to work properly and is easy for anyone without technical skills to set it up.