mu4e 1.4 brings quite a few changes, including the deprecation of
previously used variables. These are now guarded by a version
comparison. org-mu4e is not needed for linking anymore,
org-mu4e-link-query-in-headers-mode was renamed. It's new version
defaults to nil and is therefore not set anymore -- I am not sure if the
default was changed or setting it was not strictly needed previously.
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