Dear Sir,
I can setup and install best open source Zentyal, Exchange server with your brand and logo, If you need white labeled solution please chat with me and share your complete project like services your want to provide to your customers.
Zentyal Exchange have the fallowing feature.
1- Multi domain and accounts with multiple virtual domains
2- Fast and secure mail delivery
3- opensource and secure
4- beautiful web based GUI
5- easy to use
6- mange multiple domains and users