  Following up on Barabasi's recent letter to Nature [435, 207--211 (2005)], we
systematically investigate the time series of e-mail usage for 3,188 users at a
university. We focus on two quantities for each user: the time interval between
consecutively sent e-mails (interevent time), and the time interval between
when a user sends an e-mail and when a recipient sends an e-mail back to the
original sender (waiting time). We perform a standard Bayesian model selection
analysis that demonstrates that the interevent times are well-described by a
single log-normal while the waiting times are better described by the
superposition of two log-normals. Our analysis rejects the possibility that
either measure could be described by truncated power-law distributions with
exponent $\alpha \simeq 1$. We also critically evaluate the priority queuing
model proposed by Barab\'asi to describe the distribution of the waiting times.
We show that neither the assumptions nor the predictions of the model are
plausible, and conclude that a theoretical description of human e-mail
communication patterns remains an open problem.
