Was the recipient addressed according to their Exchange name or Internet address? If by Exchange name, how do these map to the current names in the Domino Directory?

If you go into Designer and design the "Sent Items" folder, you'll notice the column displays the originator, not the recipient.

You can open the standard "Sent" view and copy its recipient column, then paste it into your "sent items" folder.

I already tried that, but it doesn't work.

For migration I used PST to NSF Converter

One of the fields in the migrated document would need to contain the recipient.

Track it down, and add it to the column you're using as the recipient.

Sometimes the column can be complicated, respond with the formula if you'd like advice how to patch in, what field you found.