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An eROI study of emails sent, read and clicked on shows that the market has finally evened out into a sort of stasis where the number of emails sent out on a particular day of the week is only that which won't overload the reading rate. This has also reduced the volatility of click rates from day to day. The image at left shows this relatively fluid trend of emails sent. A contrasting graph showing fairly large discontinuities from a year ago can be seen in the jump.

Earlier volatility from Q2 '04
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Several publications, including MarketingVOX have been keeping track of optimal day-of-week research for sending emails.

Email marketers seem to be reacting accordingly, correcting for inefficiencies and taking advantage of whatever brief advantages can be gained by reacting to these studies.