Got this from the Cambodian E-visa website:
Cambodia e-Visas are valid for the same terms as tourist visas - thirty days' maximum stay or 24 hours' minimum stay, valid for 90 days from date of issue. E-Visa holders can enter through the following points of entry:
Both Cambodia international airports - Phnom Penh International Airport and Siem Reap International Airport
Two Thailand border crossings: Cham Yeam (Koh Kong) and Poi Pet
One Vietnam border crossing: Bavet (Svay Rieng)
Just to complete the circle - you can exit through ANY border point with an e-visa. An e-visa actually costs $40.
I have entered through Koh Kong a couple of times and returned through Chong Chom.