Compare these two images.   The top image comes from "Elwell's Portland and Vicinity" a guidebook to the city published in 1876 and re-published by Greater Portland Landmarks in 1975.  The major features on the postcard 1,2&3 all appear in the illustration from Elwell's.  In addition, the steeple appears in another postcard of Congress Street in this series. The one recognizable thing from our postcard is 4, which I'm pretty sure is the Baxter Building, once the Portland Public Library, now part of the Maine College of Art.

What kind of guesses did we get?