Each week we send out a newsletter to our customer base. About 75% of the time the newsletter goes out, we see large spikes in traffic in Google Analytics across our entire website, even pages not in the newsletter. This doesn't happen every time the emails go out though, and it's highly unlikely that some of the pages with the spike in traffic would be visited by the number of people Google is showing. Is this a tracking error with Marketo or Google Analytics?
There is a known spam click bot problem where spam filters check every link in the email. Do some research here on issues.
Is this a tracking error with Marketo or Google Analytics?
It's not a Marketo issue at all if you're talking about GA hits.
And I wouldn't be convinced it's a tracking "error" with GA so much as as underlying error with your links. For example, if you have a link anywhere in the email that redirects to your fallback page and an anti-phishing mail scanner visits that link, you will see a hit in GA.
GA code is a Java Script code and need a web-browser for execution. Not sure if the anti-phishing mail scanner clicks on the link will be counted as visits, do they?
Yes, any mail scanner that's doing its job will load the target of the clicked link, which itself requires JS.
How many *additional* layers of JS are loaded varies.
Note it's not true that GA requires JS. Thevariant doesn't and is included in complete implementations.