Communicating via email (vs. face-to-face) makes people less cooperative and makes them feel more justified in being noncooperative:
Two empirical studies are presented that explore how and why e-mail communication (versus face-to-face communication) influences cooperation in mixed motive group contexts. Results indicate that, relative to those engaging in face-to-face interaction, those who interacted via e-mail were (1) less cooperative and (2) felt more justified in being noncooperative. Feelings of justification mediated the relationship between communication media and the decision to cooperate or not.
Source: E-Mail Communication and Group Cooperation in Mixed Motive Contexts” from Social Justice Research, Volume 21, Number 4 (2008), 470-489