Gunnar is having a problem with Mail in macOS:
Since upgrading to Sierra, the Remove Attachments command in Mail is no longer working. It is greyed out. My Mail database is getting larger and larger. I do not want to delete the emails (for archive reasons), just the attachments.
Because it worked for Gunnar before upgrading, the most likely cause probably doesn’t apply: in Mail > Preferences > Accounts under Account Information, you can set Download Attachments to None, which might affect whether you can remove them without having forced them to download from the mail server.
Most likely, it’s a permissions problem in the nested folder that Apple uses to store attachments. That’s at
~/Library/Containers/com.apple.mail/Data/Library in the Mail Downloads folder. You can enter that path via Go > Go To Finder, select Mail Downloads, and choose File > Get Info. If your account name as “name (Me)” isn’t shown under Sharing & Permissions, or doesn’t have Read & Write under the Privilege column next to it, that’s the culprit. (Click the lock icon, enter your password, and then click + to add your account or choose Read & Write from the Privilege pop-up menu next to your name. Click the gear icon and choose Apply to Enclosed Items and confirm.)
Failing that, you might want to make sure the mailbox in question isn’t corrupt by choosing Mailbox > Rebuild.
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