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Unexplainable one-off spike in open rate when testing


Total Posts:  2
Posted: 31 May 2016 09:50 AM


There didn’t seem to be a perfect place for this question but I’d appreciate the community’s advice if possible.

For the past month or two I’ve been testing the best time of day to send to a particular email list. I’ve been doing this by segmenting the list into 8 groups and sending exactly the same email to these groups 3 hours apart. I’ve also ensured that, for example, Segment 1 isn’t sent at the same time for each send so each segment will have been sent to at a range of times throughout the day, for fair results.

All was going well, with open rates comparable to those when sending to the whole list, until the most recent send where 2 sends (sent at 12am and 3am) generated open rates 4 or 5 times that of other sends, including the rest of the sends that day. This spike seemed to also come with a higher bounce rate than normal (mainly hard bounces from Hotmail addresses with the bounce rule “Email address doesn’t exist”). The segments that were sent to at that time hadn’t seen any other unusually high open rates, neither had sending to other segments at those times previously.

Can anyone offer an explanation? They seem like genuine opens (unique opens) and CTR has benefited in the same way as open rate.

Our list is old and needs cleaning and/or a re-engagement campaign - I’ve been told not to do this by my superiors who would prefer to send to years old addresses, but I already know this is bad practice. Could Hotmail have deleted some addresses they were using as ‘spam traps’ leading to a momentary period of being seen as a more reputable sender?

I welcome your thoughts.




Total Posts:  1
Posted: 02 June 2016 01:48 AM
[ # 1 ]

Hi Andrew

I am unsure if this is the reason, because I can’t find when you sent these campaigns.
However, yesterday Microsoft Outlook/Hotmail announced it had issues with their spam filters. A lot of “spam” was being routed to
the inbox of users which otherwise would be placed in Junk folder.

It might very well be that your regular mail goes into the junk box (seeing you have low engagement lists) and by exception yesterday
your mails all landed in the inbox, resulting in much higher open rates.

Teneo Belgium



Total Posts:  2
Posted: 02 June 2016 06:33 AM
[ # 2 ]

Hi Teneo,
The send was last Thursday 26th May but I also saw the same reports about Microsoft migrating email servers to the Office 365 cloud and the subsequent problems with the Hotmail spam filters so came to the same conclusion.
Many thanks for replying.