Hello,
        A friend of mine is a die-hard user of the Windows MUA The
Bat! (yes, the client name includes the exclamation mark).  He does,
however, wish to move to Linux at some point and is looking for a mail
client which has comparable features to The Bat!

He's already tried Thunderbird and it doesn't meet his requirements,
also GPG support (either directly or through plugins) is essential.
Plus his hardware is so old that he can't just run Windows in a VM to
continue using The Bat! and Wine may or may not work, but is beig
considered.  The ideal solution, of course, is a native client which
supports the same features as The Bat!  While hardware constraints
have him considering Lubuntu as the primary candidate for a
distribution.

Those of you unfamiliar with The Bat! can refer to these pages for a
list of features (of which my friend uses most):

http://www.ritlabs.com/en/products/thebat/
https://en.wikipedia.org/wiki/The_Bat!

Does anyone have suggestions for email clients for him to try?