Mrs. Robner

Hint 1 of 4:

There's only really one suspicion that comes up against Mrs. Robner.

Hint 2 of 4:

If you get the letter delivered around 10 A.M. and read it, you'll see that
perhaps she has a lover.

Hint 3 of 4:

Show the letter to George and see what he thinks.

Hint 4 of 4:

However, there isn't any other evidence I know of.