[VM] N Stale messages are ignored

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I keep getting the above message when I save my Imap folder.

I read in an old posting that one solution is to:
- Run: vm-prune-imap-retrieved-list
- Remove the imap-cache folder and then let VM regenerate it

Is this still the best way to do this?
Or is there something/easier/better/more automatic from within vm?
Is there any meaningful risk that this will "mess" things up if I follow the
above two steps?

Also, out of curiosity, what tends to cause such stale message formation?

Thanks!