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The code is either generated by an application ("Google Authenticator" or other similar apps) or received from Google as an SMS text message, a voice message, or an email to another account.   Trusted devices can be "marked" to skip this 2-step log-on authentication. 
Email sending. An email message sent from one ProtonMail account to another is automatically encrypted with the public key of the recipient. Once encrypted, only the private key of the recipient can decrypt the message. When the recipient logs in, their mailbox password decrypts their private key and unlocks their inbox.
Another approach is to register one main email address and many auxiliary email addresses, which will forward all mail to the main address, i.e., the auxiliaries are used as aliases of the main address. The advantage of this approach is that the user can easily detect which auxiliary email is 'leaking' with spam and block or dispose it.
Email fraud (or email scam) is intentional deception for either personal gain or to damage another individual by means of email. Almost as soon as email became widely used, it began to be used as a means to defraud people. Email fraud can take the form of a "con game", or scam. Confidence tricks tend to exploit the inherent greed and dishonesty ...
A major contributor to this article appears to have a close connection with its subject. It may require cleanup to comply with Wikipedia's content policies, particularly neutral point of view.
Opt-in email is a term used when someone is not initially added to an emailing list and is instead given the option to join the emailing list. Typically, this is some sort of mailing list, newsletter, or advertising.
An iCloud account can include an email account, much like MobileMe, .Mac, and iTools did previously. However, unlike MobileMe and its previous iterations, the email account is an optional part of an iCloud account, in that the user can choose to use a non-iCloud email address as their iCloud Apple ID.
Email spam has steadily grown since the early 1990s, and by 2014 was estimated to account for around 90% of total email traffic. Since the expense of the spam is borne mostly by the recipient, it is effectively postage due advertising. This makes it an excellent example of a negative externality.