This from SULIS - Sustainable Urban Landscape Info at the U of Minn.

Herbaceous Plants with High Salt-Tolerance

Scientific name/ Common name
Aquilegia micrantha / Cliff columbine
Calamagrostis acutifolia 'Karl Foerster'/ Karl Foerster reed grass
Dianthus pulminarious 'Allwood'/ Helen Allwood pinks
Dianthus x 'Little Boy Blue' / Little Boy Blue pinks
Dianthus gratianopolitanus / pinks
Lotus corniculatus / Bird's foot trefoil
Machaeranthera xylorrhiza/ Common woody aster
Schizachyrium scoparium / Little bluestem
Waldsteinia fragarioides / Barren strawberry

Herbaceous Plants with Moderate Salt-Tolerance

Scientific name/ Common name
Artemisia schmidtiana 'Sliver Mound'/ Silver mound artemesia
Festuca 'Elijah Blue'/ Elijah Blue estuca grass
Hosta spp./ Hosta
Hemerocallis 'Stella d'Oro'/ Stella d'oro daylily
Helleborus orientalis / Lenten rose
Heuchera micrantha 'Palace Purple'/ Palace Purple coral bells
Oenothera caespitosa / Evening primrose
Sedum spectabile 'Autumn Joy'/ Sedum 'Autumn Joy'
Sphaeralcea coccinea / Prairie mallow

I love Sedum Autumn Joy. It survives beautifully year after year in a dry space in my front planting. and looks great from May thru the winter. The only problem about planting it by the mailbox is that it really doesn't get that tall. However, I think I may try it there this year! Thanks for the suggestion!
Jean