During the WWDC 2017 keynote, it was mentioned Mail in macOS High Sierra would use up to 35% less storage space. An article on thenextweb.com also says: “After installing High Sierra, Mail will begin the arduous process of compressing every message it stores. When done, old messages will reportedly take up 35 percent less space on your computer.”

However, despite having upgraded and having had Mail running continuously for over a day, I’m not seeing much, if any, reduction in the size of the “Mail” folder (inside the “Library” folder) as reported by the Info window in Finder. The window shows a total file size of 6.75GB with 6.87GB used on disk. Within a Time Machine backup from right before the upgrade, it’s 6.7GB with 6.9GB used on disk.

Why am I not seeing any reduction in the size of the folder? Is there something I need to do to get Mail to compress the messages? Or is this feature not available on all Macs?

  • Is there anyone who is seeing a reduction in the size of their “Mail” folder? I’m wondering whether compressed storage in Mail is one of the announced features that didn’t make it into the final release of High Sierra (the article I linked to might be based on one of the beta versions).
    – Rinzwind
    Sep 28, 2017 at 19:52
  • Not seeing any compression with ~/Library/Mail here. Digging into the directories you can see that all the .emlx files are still stored in uncompressed plaintext. Mail.app itself isn't advertising message compression as one of the features under "What's New" in the Help menu. Maybe the feature didn't make it in.
    – vykor
    Sep 29, 2017 at 0:25
  • @vykor: thanks for confirming you’re not seeing any compression either.
    – Rinzwind
    Sep 29, 2017 at 6:46
  • Maybe with the next update, with a couple of other things (which you'll see around on various articles). How do I check this on my machine? Sep 29, 2017 at 15:23
  • 1
    @DaniSpringer: OK, thanks for clarifying. I'm afraid that without some backup or copy of your Mail folder from before the upgrade to compare to, I'm not sure what to suggest either.
    – Rinzwind
    Sep 30, 2017 at 22:05

1 Answer 1


To not leave my own question unanswered: I haven't been able to find much information on this, so far I can only assume Mail's reduced storage space is one of the features that was announced but didn't make it into the final release of macOS High Sierra, or at least not in the initial 10.13.(0) version.

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