Just to clarify.. are you embedding Non-JS Adobe Tracking pixels on your emails? This could cause multiple concerns within your suite...
Gmail and AMPP prefetching images within emails could indeed be over counting...
First, without JS there is no user finger-printing, meaning that every time the user opens an email they would likely be treated like a new visitor and visit... (made worse by pre-fetching which again would count as a visit/visitor) and also, because this isn't using ECID service, any links that they open from those emails would be treated like a new user/visit once they reach your site.
The thing is, you can't use JS on emails.. most email readers will outright block JS because it used to be a common way to distribute malware and viruses to unsuspecting users....
What you could attempt to do, depending on your system.. is track your email opens (via non-JS tracking) into its own suite, so as to not inflate your main suite data; and on top of that, if the technology you are using to build and send your emails can use scripts (like Python) as part of the email build that could apply a unique user id (not ECID) into the tracking call so that at least the user (if not the visit) is at least identified consistently (maybe using the aid or vid params (https://experienceleague.adobe.com/docs/analytics/implementation/validate/query-parameters.html?lang...).
That is if you need Adobe to be embedded in your emails... this will take some testing to see what works...
I suggest you set up an isolated testing suite that you very carefully document all the expected hits from a few devices and attempts and correlate the data that makes it to the suite to see if it matches your expected values...
