1. He logged on to check his e-mail.

Question

2. He logged in to check his e-mail.

3. He logged on to check his e-mails.
4. He logged in to check his e-mails.
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Which ones are grammatical? Can we use all of them?

Answer 1

1 and 2 are fine, but normally the word ‘email’ is treated like a collective noun — no plural form needed.