Polite expression for saying something in short

Solution 1:

You can write "I will summarize by saying" or "I will content myself with saying"

Alternatively, you can say "To summarize" or "In summary", preceding any of these with "Therefore" if you wish.

Solution 2:

In your example, the word suffice seems appropriate.

to be enough or adequate for; satisfy.

http://www.dictionary.com/browse/suffice?s=t

So you could say:

I will suffice with saying ...

Solution 3:

"I will not repeat here the points I stated in my previous emails. I will confine myself to..."

Keep or restrict someone or something within certain limits of (space, scope, or time)

Reference:
https://en.oxforddictionaries.com/definition/confine