What the position would be if instead of posting the letter on Wed Sept 7 the sales manager had sent an email at 9am on Wednesday ?

Ogdens Ltd offered to sell a quantity of steel to Rehman Ltd for £20,000. The offer was made by email on Monday 5 September. Rehman Ltd wrote back by letter on Tuesday 6 September saying they would pay £17500 for the same quantity and if Ogdens were agreeable they must let Rehman Ltd know by Monday 12 September. In view of the high level of competition in the market, Ogdens decided to sell for £17,500. As the sales manager at Ogdens was out of the office for the rest of the week, he dictated a letter to his secretary and told her to post it on Thursday 8 September which she did.
On the afternoon of Wednesday September 7, the sales director of Ogdens heard from one of Ogdens’ competitors, Steel Supplies that they had contracted to supply Rehman Ltd with the steel. The sales director immediately rang Rehman Ltd and confirmed Ogdens would supply and sell the steel for £17500 but was told that Rehman Ltd did not want the steel as they had contracted to buy it from Steel Supplies instead.

Advise Ogdens
as to whether they have a contract with Rehman Ltd on the basis that Ogdens accepted within the time limit before 12 September and
what the position would be if the sales manager’s letter had been posted at 9am on Wed Sept. 7 and
what the position would be if instead of posting the letter on Wed Sept 7 the sales manager had sent an email at 9am on Wednesday ?