1. I received three junk mails yesterday.

Question

2. I received three junk mail yesterday.

3. I got three spam mails yesterday.
4. I got three spam mail yesterday.

5. I received many spam/junk mails every day.
6. I receive much spam/junk mail every day.
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Which expressions are grammatical? It seems that 'mail' is used as a countable noun and an uncountable noun.

Answer 1

Neither 1 nor 2 is correct. Here's the correction:
I received three pieces of junk mail yesterday.

(Mail or junk mail or spam or email -- all are non-count nouns. You need a count noun in there so that the numbers and "many" make sense. http://grammar.ccc.commnet.edu/grammar/noncount.htm )

Make corrections in 3 - 6, and I'll check them for you.

Answer 2

1. I received three pieces of junk mail yesterday.

4. I got three pieces of spam mail yesterday.

6. I receive much spam/junk mail every day.

(Thank you for your help.)

Answer 3

With the word "spam," you don't need to attach the word "mail."

These mean the same thing:
~ spam
~ junk mail