I'm wondering if it is "I have sent a mail to him" Or "I have sent mail to him"
I could not find much online. Thank you.
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As some of your initial comments explain, we can say:
"I've sent a package to him, a letter to her, a parcel to Fred, and a telegram to Phil."
I can even say, "I sent a piece of mail to him," but, if it's just mail, there is no indefinite article:
I have sent mail to him.
However, for some reason, email works with an article (or without, particularly if it's more than one message):
I sent him an email.
"Mail" is an uncountable noun, so you must say, "I have sent him mail," not "I have sent him A mail." It's similar to saying, "I have eaten rice," instead of "I have eaten A rice." As mentioned in the previous answer, if you use the word "letter", then you not only can but must use an article. You would say either, "I have sent him a letter" or "I have sent him the letter" depending on whether your interlocutor were previously aware of the existence of this particular letter.