The recipient needs access to the Internet to receive email.
Viruses are easily spread via email attachments (most email providers scan emails for viruses on your behalf).
Phishing - sending an email to a user falsely claiming to be a legitimate company to scam the user into providing information, such as personal information and bank account numbers on a bogus website. The details will then be used for identity theft.
No guarantee the mail will be read until the user logs on and checks their email.
Spam - unsolicited email, ie junk mail.