Bounce e-mail (sometimes referred to as bounce mail) is electronic mail that is returned to the sender because it cannot be delivered for some reason. Unless otherwise arranged, bounce e-mail usually appears as a new note in your inbox. E-mail users can encounter bounce e-mail because an addressee has changed his or her address, because their mail box is full, because the note is misaddressed, or for some other reason.
Hard and Soft Bounces
A hard bounce is an e-mail message that has been returned to you because the recipient's address is invalid (permanently undeliverable). A hard bounce might occur because the domain name doesn't exist, the recipient is unknown, the address is invalid (typos or changed address), the e-mail recipient's mail server has blocked your server, or there's some type of network problem on the recipient's end.
A soft bounce is an e-mail message that gets as far as the recipient's mail server (the address is recognized), but is bounced back before it reaches the recipient. A soft bounce might occur because the recipient's mailbox is full (over-quota), the server is down or overloaded, the message is too large or the user has abandoned the mailbox. Most e-mail service providers will attempt to deliver the e-mail regularly for a few days. If it is still undelivered, it becomes a hard bounce.
User Unknown bounce occurs when the destination e-mail box no longer exists, has been shut down or if the domain portion of the e-mail address is not valid. This typically happens when someone leaves a company or changes to a different ISP or when there is a typo in the e-mail address. If you receive a user unknown bounce, and you have checked to make sure the address is correct, the next thing you should do is try to contact the person you were trying to e-mail by other means. Frequently, people are not aware that they are bouncing e-mail until someone else tells them.
Mailbox Full bounce also known as 'Recipient Over-quota', '<recipient>'s mailbox is full' or 'Mailbox disk quota exceeded' indicates that, even though an e-mail address is good, the subscriber’s mail server rejected the e-mail. This is because most e-mail servers and mail hosting providers set limits on the amount of e-mail that will be stored in an account. This error will stop as soon as the recipient makes additional room in their mailbox (usually by removing old messages from the server), so you should probably resend your message a little later. However, especially with free accounts, this message could actually mean the user no longer checks the account.
Message exceeds size limit bounce means that the size of your message, including all headers, text and attachments, exceeds the domain's maximum per-message size limit - essentially, that your e-mail is too big to be accepted. You should try to reduce the size of the message, or try to split the e-mail into smaller parts and resend it.
Domain Not Found means that the domain name to which you sent the message does not exist. Usually, this means you misspelled the domain name, but it may indicate a problem with the domain's record that prevents the domain from being found. SPAM - System administrators frequently set up their systems to refuse mail from spammers, but since no spam filtering system is perfect, your message may have been caught in the spam block. It means that your provider's domain name is explicitly listed as a known spammer on a blacklist. This may be based on an external service that provides blacklists of known spammers to ISPs, or the administrator may have placed a block based on a large volume of mail coming from your domain. Most of the time, your e-mail provider will need to contact the system administrator to have the block removed, so you should contact your provider immediately.
Too Many hops indicates that mail is probably caught in a routing loop. Either an alias for a local user points to a recursive alias on another system, or two systems are using each other in their MX records. There are other possible causes for this error, although these two are the most common culprits.
Connection Timed Out also known as 'Connection Refused' usually results from high volumes of mail being processed at the time your message was received. This could be due to the server receiving more mail then it is used to, a external attack on a domain or an internal setup problem, causing the domain's mail servers to refuse connections or cut connections before a message is fully sent. Mail exchangers are set up to only accept as much mail as they can handle, so when problem is resolved, you will be able to send your mail without problem.
Relay Access Denied occurs when a domain has recently changed hosts, and while the change has taken place, the new domain record has not yet propagated fully, and your message reached the old hosting company which no longer accepts mail for the leaving domain.
Residential IP address means addressee server doesn't accept messages sent from a residential IP address. A residential IP address (either fixed or dynamic) is the one any DSL, cable or modem user get when connecting to the internet. Basically, that means the addressee server doesn't accept e-mails from those users using their own in-house SMTP server. AOL is one of those servers: http://postmaster.aol.com/guidelines/standards.html. For more info on this topic read: "Using your own SMTP server, advantages and drawbacks".
Unknown - While many bounce messages are returned with clear and concise reasons why delivery has failed, there are unfortunately many non-standard and unique ways in which e-mail servers will bounce messages. When e-mails are bounced because of “unknown” reasons, the e-mail address may or may not be bad and/or the e-mail may or may not have been received. Sometimes, this type of bounce is due to a temporary failure on the Internet (e.g., a mail server is down, a connection is lost, etc.).