When the user hits the BACK button within the framework of sqwebmail:
1) Does the browser look only in a local cache for the previous page? Thus Apache and sqwebmail could not be responsible for the curruption.
2) Or does sqwebmail still get invoked in someway. For example, maybe sqwebmail does a re-authentication when going back to the message list?
That depends on the browser.
SqWebMail explicitly sends headers that direct the browser and any intermediate web caches not to cache any content for any reason.
A properly-behaving browser should try to re-fetch the older URL.
What actually happens in your case is something that only you can figure out.
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