In the physical postal service, a piece of mail which cannot be delivered to the addressee may be returned to the sender. If you send an e-mail which cannot be delivered, an electronic postmaster sends an automated e-mail to you informing you the delivery failed. E-mail delivery can fail for several reasons.
Any misspellings in a recipient's e-mail address can cause the email delivery to fail. Common spelling or typing mistakes include typing a period "." at the end of an e-mail address or using the wrong suffix on the email address -- typing ".com" instead of ".net," ".co.uk" or ".gov," for example.
The recipient's e-mail service may -- sometimes incorrectly -- identify your e-mail as spam. If the recipient's spam settings are restrictive, messages from an unknown e-mail address may be returned undelivered. Making sure the recipient already has your e-mail address in her contact list should prevent non-delivery in this instance.
Many e-mail providers place a limit on the storage space in a person's e-mail inbox. If an e-mail inbox becomes completely full and has no more space for new messages, incoming messages may be returned to the sender with a postmaster delivery failure message.