Get rid of the popup when long-pressing an item in the message list and select/deselect the item instead.
This should probably be implemented together with:
Related: #3555 Allow reading messages in select mode
Not sure this applies to 5.7xx, but it's gotten especially bad in this release.
Yes, where are these check boxes to select several messages gone?

I'm pretty sure the pre-Material UI allowed users to swipe to select/de-select. I miss that feature.
Can you make this configurable? I rely on the context menu popup and I find it much more important than being able to mark the messages.
Or how can I access the "More from this sender" menu point?
"More from this sender" is no longer available. See also the description for PR #4608.
There are no plans to bring back the context menu. To learn why the answer to "can this be an optional feature" is almost always "no" read http://neugierig.org/software/blog/2018/07/options.html
"More from this sender" is so useful and I use it daily. Can this be implemented at least in the message view - for example by long press the sender address instead of copying it to the clipboard?
Hey, just tried out the new release, thanks for the new select behavior! Just one question: could it be implemented, so that holding and dragging up/downwards selects multiple messages at once?
@espectalll: Please open a new issue for that feature request.
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Not sure this applies to 5.7xx, but it's gotten especially bad in this release.