Hi both,
As Sanford indicated, this is indeed behaviour that is all too common. And with the numbers you indicate one large email server upgrading their features to include a mailscanner can impact a relatively large part of your audience.
Unfortunately the addition of the "Visited web page" is not guaranteed to mitigate the issue, as some mailscanners will actually mimic the actual web visit too.
Hey Emilia,
Following up to my previous response. I saw today that Marketo announced the release of some upcoming new features and one of them involves filtering out email bot activity. I don't know details on what that entails but I think it's a step in the right direction!
Here are the release notes: https://docs.marketo.com/display/public/DOCS/May+%2721
Caroline
Noticed that one too. Eager to find out how they approach this.