I'm new to XenForo so I may have missed something but so far I haven't found any email digest script or add-on comparable, for example, to Mark Hamill's phpBB Digest Mod. There is @netkingZ's xenDigest but that serves a different purpose and isn't open-source. There was the Daily Digest Email System script by @smoseley which might provide a starting point but it isn't being maintained nor is it an add-on.
I've seen @TigerXtrm offer to pay for such a digest and @Neutral Singh ask for info on digests but get no apparent response – which is a bit discouraging.
Nevertheless, having such a digest is really important for many forums. I see that Discourse, the open source forum software that is coming on strong and likely to provide XenForo with serious competition in the next few years, has such a digest built into core.
I'm not knowledgeable enough with XenForo to be the lead programmer, but I'd be willing to be part of a team.
If you either would like to have such an add-on or would be willing to be part of creating such an add-on, please reply to this thread and lets see what we can do.