Although there are many reasons why an email could be returned or undeliverable, the most common reason is because the email address for the recipient is incorrect. Double-checking the accuracy of the spelling and the domain name can help prevent the email from being returned.
If the sender is attempting to send a bulk email, it may be returned if too many email addresses are attached. Some email services limit the amount of email addresses. As a result of this limitation, the service may bounce the email back if the sender exceeds this amount. Another reason why an email may bounce back is if the recipient's mailbox is full or if a file attached is too large for the recipient's mail server. Sending a shorter message without the attachment helps test this theory.
If an email is hacked into by a spammer who sends emails to multiple recipients, it may also come to the sender's inbox as returned or undeliverable. According to the Computing and Educational Technology Services at Penn Engineering, spammers often use a real email address to reach more recipients and a few may bounce back to the sender's inbox. If this happens, change the password to secure the email account.