Relative clause separation

It does sound odd. Perhaps there is some technical grammar reason which is beyond me.

The first sentence tells me about one letter, which could have been one of many letters that arrived, but that one was addressed to Mary.

Using 'the' in the second sentence means either of two things to me.

1)Only one letter arrived yesterday, in which case I would say,

The letter that arrived yesterday was addressed to Mary.

Possibly one or several letters arrived yesterday, but we are talking about a particular letter for Mary, in which case I would say,

The letter, which was addressed to Mary, arrived yesterday. or
The letter addressed to Mary arrived yesterday.

The first emphasises the receipt of the letter, and the second empasises that it was for Mary.

Nona and Matthew

Your replies are very helpful. The literature said that grammarians call this kind of clause's rightward movement as extraposition. When the subject determiner is 'a', we can move the restrictive relative clause but when it is 'the' we can't. I didn't still grasp the reason but anyway what Matthew felt proves it is true. Thank you.

paco