Mail is letters, parcels and the system that carries them — "the mail arrives before nine", "I'll mail it tomorrow". Male describes the sex that produces sperm, or a man or boy — "a male blackbird", "two male applicants". They are pure homophones with no shared meaning: if it can be posted, it is mail.
The core difference
These two never overlap in sense, which makes this an entirely mechanical choice — and precisely why the error slips past unnoticed, since both spellings are real words that no spellchecker will query.
Mail works as a noun (a pile of mail), as a verb in American English (mail the forms back), and as a modifier (mail order, mailbox, mailing list). Male works as an adjective (the male line, male-dominated) and as a noun for a person or animal of that sex (the males defend the nest).
The test you can run
Ask whether the thing could be delivered. If yes — envelopes, packages, messages, a postal service — it is mail. If instead you could substitute man, boy or not female, it is male.
For a spelling hook, note that mail shares its ai with airmail and claim — things sent and received — while male shares its ending with female, the word it always sits beside on a form.
British and American differences
The verb is the main split. Americans mail a letter; the British, Irish and Australians post it, and use mail mostly as a noun or in fixed names such as Royal Mail. So I'll mail you the contract reads as American; I'll post you the contract reads as British. Both are understood everywhere.
Email — now usually written closed up, without the hyphen — is universal, and its verb form is standard in all varieties: email me the file.
The edge cases
Two uses of mail have nothing to do with the post office, and both are correct:
- Mail as armour: chain mail, a coat of mail. This sense survives mainly in history writing and fantasy fiction.
- Blackmail, which is a single fixed word and takes the mail spelling.
On the other side, male has a technical sense in engineering: the connector that plugs into another is the male one, the receiving side is female. This is standard terminology in electronics and plumbing, so a male USB connector is not a typo.
Finally, do not confuse either with mall (a shopping centre) or maul (to attack), which sound close in some accents.
