The speaker is amazed that an old woman is still alive, and the proof arrives in the mail. She is a correspondent, someone who writes letters, and the poem is about receiving one. The phrase "a matter of wonder" sounds quiet but sincere: a person who has lived a hundred years is still reaching out across distance.
What moves the speaker most is the physical letter. Her age "finds" him in her handwriting, as if the century itself were travelling inside the envelope. The "venerable ink" and "plain uncut paper" suggest something simple and unhurried, with no decoration and nothing wasted. Judging by these details, her writing feels both old-fashioned and deeply personal, a trace of her hand that a typed message could never carry.
The mood is tender and reverent, with a hint of gratitude. The speaker doesn't explain his feelings at length. He lets the ink and paper speak, and the short, spare lines mirror the plainness he admires.