WASHINGTON (AP) — Before there was home mail delivery, slaves often carried letters to and from U.S. post offices or directly to recipients themselves.

Now the Smithsonian’s National Postal Museum is recalling that piece of postal history in a new exhibit opening Thursday. “Freedom Just Around the Corner: Black America from Civil War to Civil Rights” is the museum’s first exhibit devoted to African-American history.

The new exhibit features letters carried by enslaved Americans and mail sent by and to leaders of the civil rights movement.

One letter carried by a slave named Susan in 1850 contains a message about how Susan should be sold to a slave dealer in Richmond, Virginia. Curators say Susan probably wasn’t aware the letter she carried to a post office contained arrangements for her to be sold.