The Weekend Effect
Sending mail only Monday through Friday can cause reputation and delivery problems at some ISPs, even when senders are doing everything right. This “weekend effect” is a consequence of how ISPs measure reputation over time.
Most ISPs calculating complaint rate use a simple calculation. They measure how many “this is spam” clicks a source IP generates in a 24 hour period. Then they divide that number by how many emails were delivered to the inbox in the same 24 hour period.
The weekend effect happens when a sender sends on weekdays and not on the weekend thus lowering the number of emails delivered to the to the inbox. Recipients, however, still read mail on the weekend, and they still hit the “this is spam” button on the email. Even if the number of “this is spam” clicks is lower than a normal weekday, with no incoming email the rate of spam complaints goes above ISP thresholds. Even a very well run mailing program may see spikes in complaint rate on the weekends.
Now, when the ISPs are measuring complaint rates over time, they take the average of the average complaint rates. If the rates spike high enough on the weekend (and they can spike to the 1 – 3% range, even for a well run list), that can hurt the senders’ reputation.
The good news is that ISPs are aware of the weekend effect and take this into account when manually looking at complaints. The bad news is that not all of the major ISPs take this into account when programatically calculating reputation.
There isn’t very much senders can do to combat the weekend effect, except be aware this can happen and may be responsible for poor mailing performances on Monday. If you are seeing delivery problems you think may be a result of the weekend effect you can contact the ISPs and ask for manual review of your reputation. Some ISPs can provide manual mitigation for senders with otherwise clean stats. d