Why does Watson visit the Grimpen postmaster? What does he discover from this meeting?

Watson visits the Grimpen postmaster to find out if Barrymore directly received the test telegram. He discovers that the telegram was given to Mrs. Barrymore, rather then her husband.

My boy here. James, you delivered that telegram to Mr. Barrymore at the Hall last week, did you not?”

“Yes, father, I delivered it.”

“Into his own hands?” I asked.

“Well, he was up in the loft at the time, so that I could not put it into his own hands, but I gave it into Mrs. Barrymore’s hands, and she promised to deliver it at once.”

“Did you see Mr. Barrymore?”

“No, sir; I tell you he was in the loft.”

“If you didn’t see him, how do you know he was in the loft?”

“Well, surely his own wife ought to know where he is,” said the postmaster testily. “Didn’t he get the telegram? If there is any mistake it is for Mr. Barrymore himself to complain.”