Generation of customized response messages is described. According to the method, an incoming communication request is received and a relationship between an initiator of the communication request and a recipient of the communication request is determined. Based on a number of calendars for the recipient, an availability of the recipient is determined. When the recipient is unavailable, a customized response message is generated. The customized response message 1) is based on the relationship between the initiator and the recipient and 2) indicates an estimated response time for the recipient based on the availability of the recipient.