To encourage recipients of electronic messages to respond in a timely manner, certain messages can be designated as time-critical messages. Such messages are delivered with an allowable response time attribute. If the recipient responds to the message within the allowable response time, nothing happens. If the recipient fails to respond within the allowable response time, the functionality of the recipient's system is altered in a way that gives the recipient an incentive to respond. For example, the recipient's system may be inhibited from receiving any further messages until the recipient responds to the time-critical message. Embodiments are described for implementing time-critical messaging in e-mail, instant messaging and voicemail systems.