There are several reasons why email is sometimes undeliverable:
- the recipient does not have an email account on the mail server (the email address is invalid)
- the recipient is over his/her email quota and is unable to receive any new email until his/her inbox is cleaned out
- the email size is greater than the maximum size the recipient's mail server allows
- the mail message inadvertently gets caught in spam/virus traps or junkmail/other filters
Undelivered mail does not always automatically get delivered when the problem clears up, so be sure to send your email again at a later time after you have verified the recipient's email address.