To anyone sending emails (including posterous), I'd recommend going beyond A/B tests. You should know the optimal time to contact each user.
For new users you can intentionally send emails at various times (and days of the week). Then you can track a time-specific clickthru rate for each user. Eventually you'll converge on one or more optimal times/days. You can accomplish this with a machine learning algorithm or you can simply track the timestamp of each user's last N clicks and use that to decide when to email them. Precision isn't of the essence here, but I've found that clickthru rates can be dramatically affected by picking the right time of day and day of week.
You need an awful lot of clicks to get time-specific CTR, and that strikes me as unlikely with most services, but it takes zero clicks to identify one time the user is at the computer ("the anniversary hour:minute of their signup") and one click to identify a time at which they open the email (the time at which they opened any email from you, causing a download of your web bug, or clicked from the email to your website).
Silverpop offers this as a feature, though I think the algorithm is pretty naive. Basically, it aims to send around the time you normally open messages.
May be it's just me that I'm not so impulsive buyer as Sachin. But here is what I do when I see the Groupon deal - ponder upon if I want to buy this or not. Of course, the % discount is always incredible, there is no question about it, so I know I'm going to get it at lot cheaper price than I usually do. But the pondering is always about - do I need it? So the moment I see a Groupon deal, I can't decide right away that I want to buy it or not. I keep pondering over it till afternoon, and most of the times I purchased the deals in the late afternoon or evening. So it takes some time for me to see the deal, and then make the decision. I can't think of myself making the decision within few minutes after I see the deal, and that too before sleep after a tiring day of work. So I actually don't mind getting these emails in the morning, when I'm fresh and ready to get started for the day. So not sure if the pattern Sachin wrote about is most common or the pattern I'm seeing is common. Yeah, that's when A/B testing will help.
I'm a very impulsive buyer. If I see something I like (or is a good deal), I just do it. You should see my Amazon order history, it's been pretty insane since I got Prime :)
Ironically, the "Daily Posterous Subscriptions" email I receive every day suffers from exactly the same issue that their co-founder has observed in another service.
Because it always seems to be sent around 4:30am and is relatively low value, it's one of the first emails I delete during my morning inbox routine.
For new users you can intentionally send emails at various times (and days of the week). Then you can track a time-specific clickthru rate for each user. Eventually you'll converge on one or more optimal times/days. You can accomplish this with a machine learning algorithm or you can simply track the timestamp of each user's last N clicks and use that to decide when to email them. Precision isn't of the essence here, but I've found that clickthru rates can be dramatically affected by picking the right time of day and day of week.