The frequency conundrum

What is the perfect frequency to send mail? Is it daily, weekly, monthly, hourly, minutely (is that even a word?) or randomly? Any number of experts will give you a definitive answer to this question, but I don’t believe there is a single answer.
The frequency recipients will respond to depends on the type of mail, the recipient expectations, the sender and a host of other factors.
For one example look at the mail sent by social networks. Many people, myself included, will accept dozens of emails a day telling me someone wrote on my Facebook wall or retweeted something I said or wants to link to my network on LinkedIn. Another example is when I’m traveling or waiting to pick up someone who is, I am thrilled to receive multiple updates an hour from the airline.
This willingness to receive frequent commercial or bulk emails doesn’t necessarily translate to marketing emails. When Sur la Table started sending double digit amounts of email a week, I down-subscribed, and had they not let me pick an acceptable-to-me frequency I would have unsubscribed completely.
A lot of marketing experts insist that mailers don’t send frequently enough. That increasing frequency increases ROI. What a lot of people miss are all the caveats in the fine print. In their minds, increasing frequency goes hand in hand with increased segmentation, targeting and recipient specific emails.
The idea isn’t simply to mail the entire list more frequently but to mail those who are more open to increased frequency. This is an idea I wholeheartedly support.