How do you determins if its plural
"He sends email." was marked as incorrect.
'he sends emails' is accepted. In singular form you would need to add an article
Not necessarily. Email can refer to a category as well as an item. You would not say "He sends mails." or even "He sends a mail."
but it's not categorical in this context; that would be more like 'he communicates via email'.
For mail, yes it's categorical, but the item is a letter. For email, the item is also an email, which is what is being referred to in this sentence
So, does the Swahili indicate that it is an item rather than a category?
The voice is wrong