Angela Ridpath Sounds like it could be an email content scanner in the recipient mail server. Essentially this is something in the server receiving the emails that analyzes all content in the email as part of analyzing whether it's spam content. That usually includes checking hyperlinks for destination URLs and downloading images, which is how the open activity is recorded (when the single tracking pixel is called from the Marketo servers, that's how we know the email has been opened).
Since it's something added into the recipient mail servers by the admins of those servers, that would explain why that activity is showing up only for that one company and not all.
Kiersti Esparza wrote this blog Understanding a Spike in Click Activity that helps explain how this works.
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