It's oftentimes better for both parties when rejecting someone to do it via email. The person being rejected isn't put on the spot to compose themselves and be

It's oftentimes better for both parties when rejecting someone to do it via email. The person being rejected isn't put on the spot to compose themselves and be a good sport while the person doing the rejection can manage the response from the person getting rejected (don't have to immediately answer follow ups and/or defend their decision).