Postal service actually resumed (with the help of the Vietnamese) in 1979. This 4v stamp set was certainly sold in Cambodia and was used as well. In 1980, civilians were still suffering from aftershocks of Khmer Rouge and they were too scared to make immediate foreign contact. However, Vietnamese and international aid workers did use the mail service. Here's a sample of 1980 outbound mail, from Phnom Penh to Ho Chi Minh City (Saigon).
Spek mentioned that mail was via Vietnam. International mail in and out of Cambodia was all via Vietnam in the first half of 1980s because of sanction. Later arrangement was made with Thailand but the route was limited.
