Originally the Table of Contents in our emailed Digest would link to the full post that was included further down in the body of the digest. However, many email readers, including Apple's have stopped supporting "internal" linking (linking to a different part of the same email).
To compensate for this, I first removed the table of contents when we send you a mobile-formatted digest. We send you a mobile-formatted digest, by default, when the last time you read a digest it was on a mobile device. Besides the lack of the table of contents, it also includes mobile friendly banners (square-ish, rather than wide ones).
We've now changed the default for desktop formatted digests (which we send by default if you last opened your digest on a desktop computer), so that the table of contents links to the thread on the website (to the first post that is included the that day's digest, but on the website). This should restore the table of contents functionality for all email readers.
You can override both of these defaults by going here and scrolling down to the sections "Digest Format:" and "Digest Table Of Contents Linking:"
And don't forget you can customize which categories are included in your digest and which ones are not. Click here to see how.