III Conditional/II Conditioanal

Is this sentence right: "I would not have opened the mail if it had contained a virus."
So should I use III Conditional in the sentence?
Or I can use II Conditional?: I wouldn't open the mail if it contained a virus"?
Or it depends on the article before "mail".
E.g. I should use "a" in the 2nd sentence because the mail is not defenite? It is just a mail.
And I should use "the" in the 1st sentence because the mail is defenite?
Can you explain it me?)