It is a temptation for email marketers to begin to send out more offers to customers if current campaigns are working well in the hope that increased frequency will yield greater returns.
This can be a risky strategy though; while more emails may produce better results, there is a point at which customers will tire of too many emails and start to unsubscribe, ignore, or mark emails as spam.
How many emails constitutes too many is difficult to judge and depends on the recipients, the content in emails, and other factors. I would say ten emails in eleven days, as in this example from VistaPrint, is too many, unless you are especially keen on receiving offers for free business cards.
However, there is also a risk in sending so few that recipients forget they signed up in the first place.
Mark Brownlow has produced an excellent summary of the issue, complete with some useful stats and tips for finding the right balance, as well as this 'frequency curve', which illustrates the problem well; sending more can increase profits, but there is a point when too many emails will have the opposite effect:
The key here, according to Brownlow, is to work out where you are on the curve. If you're at point A, it might be worth cranking up the frequency a little, but at D, then reducing the number of emails sent out could have a positive effect.
He cites the example of toptable.com, taken from an Emailcenter white paper, which halved the amount of emails and found that the total number of bookings went up.
The key here is to respond to customers and let them decide how many emails they want to receive in the first place, or only upping email frequency when it makes sense to do so, around Christmas for instance, or based on the information you already have about the customer.