Gmail marked all emails ( Issue no 15 ) as SPAM. Many subscribers brought it to our notice.
Why ? Our guess is
Massive unique URL generation
To track which subscriber has clicked on which article/project/tweet we made every url unique. We do that so we can recommend and rank the articles for subscribers as per their interest. To track clicks each URL is unique. This results in massive URL generation. Something Gmail SPAM Filter detects and find unusual.
We decided to stop generating recommending content as of now so the emails wouldn't be marked as SPAM. What we would do instead is give each subscriber a preference page and allow them to click on topics that interest them. For e.g. a Flask Programmer would like to see more of Flask content as oppose to Django. So he could click on Flask under web framework category and we would only send him Flask related content.
The fact that gmail make for above 60% of our subscribers forces us to take this step.
Let us know if you have any alternative we could try