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251 – The recipient is not local to the server, but the server will accept and forward the message.
The good news is that the SMTP error messages you need to worry about as an email sender are really primarily limited to just a few – those in the 5XX range – and of those, the ones that you really need to worry about are the ones in the 55X range – i.e. Of these, by far the most common one – the one that as a sender you will see most often and to which you must pay immediate attention – is the 550 SMTP error code, which can range from “user not found” to “mailbox unavailable” to any number of other similar variations, but which for you, dear mail sender, should almost always be read to mean “remove this email address from your mailing list immediately.” (On the other hand, your mail server administrator should care deeply about many more of these codes, however we assume that if they are administering your outbound mail server, they are already familiar with them.) So, back to that 550 error code.
450 – The requested command failed because the user’s mailbox was unavailable (such as being full). 451 – The command has been aborted due to a server error.
Think about this: in this day and age of nearly unlimited email storage (nearly all ISPs now offer multiple Gigs of email storage), just how inactive does a user have to be in order for their inbox to fill up?
There could be a lot of reasons, including that someone entered it wrong, or that someone intentionally entered a fake email address into your system (such as when you require a user to divulge an email address in order to receive a download, etc.).
Regardless of how it ended up on your mailing list, if you send a mailing to it, it tells the receiving system one sure thing: that you don’t confirm email addresses before adding them to your mailing lists.
Anybody who sends email has seen them, in one form or another – those SMTP error codes, often returned in bounced email, such as “550 Requested action not taken: mailbox unavailable” or “550 5 2 1 mail from refused spam site.” These are often in response to SMTP commands that have ‘gone wrong’ between your email server that sent the email, and the receiving email server that is unable to deliver it (or refuses to deliver it) for some reason. Unfortunately, many receiving systems seem to mix and match these error messages, rather than adhering to the prescribed code messages.
(‘SMTP’ stands for Simple Mail Transfer Protocol.) First, here is a list of SMTP codes and error codes, and what each message is supposed to say and mean: 211 – A system status message. (on their side) 452 – The command has been aborted because the server has insufficient system storage.The bottom line is: why hang on to an email address that belongs to a user who will never be a positive asset for you, and can only cause you trouble?So the next time an email that you send bounces back, take a good look at the information in the bounce message, and take the appropriate action based on that message. If you are actually receiving bounce messages that include “mail from refused spam site” or a similar message, the odds are good that your mail already has been blacklisted, and you’ll need to deal with that immediately.The following recipient(s) could not be reached: Recipient: [SMTP:[email protected]] Reason: Remote SMTP Server Returned: 554 5.7.1 Service unavailable; Client host [220.127.116.11] blocked using sip.rbl.I assume that it is on their end as I receive the same message whether I send via my desk top or smart phone.