Using relative clause in the middle.

Hello my friends I hope that you are all having a good day.

I have two question now.

1-) Almost %99 of examples on the Internet have a relative clause at the end of sentence.

Examples are almost the same.

(Subject + verb + object+ adjective clause) or (Subject + adjective clause + verb + object)

They are very easy but I am interested in hard ones.

Firstly, let's define a main sentence.

" I received an e-mail from Harvard. "

If I would like to add information about "an e-mail", I have to put it at the middle.

But how about "comma" ? According to rules, we should not use a comma when using defining relative clause.

"I received an e-mail that I have been waiting for a long time from Harvard."

It is not good because "from harvard" seems to belong to the clause. Actually it belongs to the main sentence.

I want to put a comma like " I received an-email that I have been waiting for a long time, from Harvard."

but I am not sure whether it is true or not..

Always the same problem and I am really fed up with them.

Another one,

"I am going to the same market that I went to yesterday, to buy a few apples."

I feel that comma is necessary because when there is no comma, it seems: "I went to the same market to buy a few apples." But actually it didn't mean it. Maybe yesterday, I bought something different...

Thank you very much