Mailbox Not Found, invalid mailbox, User unknown, not our customer: these are all saying the same thing. A couple of common reasons could be:
* You mistyped the email address. The single most common reason is simply that you made a typographical error in the email name. Check the entire email address for an error. Computers are very picky, and with the exception of upper/lower case email addresses must be exactly correct.
* It’s an old address that’s no longer in use. Perhaps the person you’re attempting to email has changed their email address and you’re using an old one which is no longer valid. Make sure what you’re using is current.
Host unknown, Domain Lookup Failed: this means that the mail server you’re attempting to use, the “example.com” part doesn’t exist. A common reason is, once again, a typo on your part. Make sure you typed it in exactly correct. Another reason are ISPs that change their name and eventually stop supporting the old name.
Unable to Relay: “Relaying” is when you send email to one mail server, expecting it to pass the email on to its intended recipient who resides on a different mail server (known as “non-local email”). A mail server will produce a “Relaying Denied” error when an unauthorized user attempts to send non-local email.
Mail servers will not relay mail from just anyone. Otherwise, spammers could flood the mail server, making it do their dirty work. You must be authorized to use a mail server for relaying. Unauthorized users will always receive the “Relaying Denied” error.
* The “From” address might not match an account on the email server.
* The ISP might require that email comes via a connection (dialup or DSL) actually provided by the ISP – sending using someone else’s connection might not be allowed.
* The ISP might require you to authenticate before sending email and you haven’t.
* A mail server somewhere could be misconfigured.
There’s no blanket answer if “unable to relay” happens only occasionally. Double check the email address you’re sending to, for starters.
Temporary Errors: errors like “no adequate servers”, “Connection Timed Out”, “Resources temporarily unavailable.”, “Out of memory” all typically indicate a problem with a mail server that you probably don’thave any control over. They are, in general, temporary, and should resolve themselves over time. Look carefully at the bounce message; the email server involved may continue to automatically try to deliver your email without any action required on your part.
Blacklist Filters: If you see messages that indicate your email was “blocked”, or “listed in”, and references to sites that have things like “spamcop”, “dynablock”, “blackhole” and similar in their names, then your email was probably intentionally blocked because the receiving system thinks your ISP’s mail server is a source of spam.