1. An electronic messaging system comprising: at least one processor; and at least one non-transitory computer-readable storage medium encoded with executable instructions that, when executed by the at least one processor, causes the at least one processor to perform operations comprising: receiving first electronic messages that are addressed to a particular user and that are from multiple, different senders; monitoring for responses by the particular user to the first electronic messages; based on monitoring for responses by the particular user to the first electronic messages, determining response patterns of the particular user by sender; based on determining response patterns of the particular user by sender, storing, in electronic storage, response pattern data for the particular user by sender; after storing, in electronic storage, response pattern data for the particular user by sender, receiving second electronic messages that are addressed to the particular user and that are from multiple, different senders, the second electronic messages being different than the first electronic messages and being received subsequent to the first electronic messages; and alerting the particular user to electronic messages determined to be important based on past response behavior of the particular user by identifying a subset of the second electronic messages to which the particular user has not yet responded and that are each from a corresponding sender for which the stored response pattern data indicates a threshold response rate by the particular user to past electronic messages from the corresponding sender and, when the particular user begins using the electronic messaging system, causing display, in a list that only includes electronic messages to which the particular user has not yet responded, of the identified subset of the second electronic messages.