Importing Mail after new HD install and recovery via Time Capsule

That means that one or more corrupt messages is in a "Sent Messages" mbox folder. Referring to your first screenshot, it looks like you have two such mailboxes. Move them both to the Desktop, then launch Mail again and see whether it succeeds in building the index. If it does, you can try to import one of the mailboxes you moved (File > Import Mailboxes...). The imported messages will be in a new mailbox, and you can then move them wherever you want.

If successful, try importing the other mailbox.

At some point, you'll probably run into an error. Then you face a tedious process of elimination. Make a copy of the mbox folder. From the copy, delete about half of the thousands of emlx files it contains, each representing an individual message. Try the import again. If that succeeds, try to import half of the remaining messages. Continue in this way until you've isolated the corrupt files.