Define lost?!

Mr T posted this letter last week as we want to reject the blinds fitted by John Lewis under the Consumer Rights Act, but after a week the letter has not been delivered.  He was given a phone number to call by the Post Office which he duly rang.  The lady told him our letter wasn't lost, so where is it he asked.  I don't know she said, it's just delayed.  It's not lost until the 18th April.  Hmm, sounds lost to us!