Is it possible to show multi-level threads in Mail.app?

Solution 1:

I'm sorry; that's not possible.

Solution 2:

As an alternative to the lack of Mail features (good threading being one of those), you can try any of the following, which I’m sure you already know:

• Postbox: Lots of features, multiplatform, not free.
• Thunderbird: Very decent mail client, free and also multiplatform.
• Mailplane: If you like Gmail but don’t want to lose OS X features, Mailplane is a very decent alternative combining the best of gmail with a respectable OS X integration.
• Sparrow: A new alternative (only for gmail) that is growing in popularity for its ease of use and iPad-like interface. Although it only works for gmail, the plan is to expand it. Still very beta. Still free. Worth to follow around and check every now and then.
• There may be others but these are the most known/used (excluding Mail.app of course). Eudora is also popular, but I believe it’s a little bit outdated.