One of the most common email related support calls I get is someone wondering why they’re receiving “Undeliverable Mail Returned to Sender” notifications when sending out email to a particular email address. These “bounce back” messages will tell you why the email couldn’t be delivered, but often times the language used isn’t immediately understandable by someone who isn’t an IT professional, or “into computer stuff”. I thought I’d go over some of the more common undeliverable errors, and explain what they mean. I made a test email address for this purpose, blogtest@iphouse.com – and then sent several emails to it that were intended to be bounced for various reasons. Here’s the first bounce back message:

This is the mail system at host smtpin-2.iphouse.net.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
<blogtest@iphouse.com>: permission denied. Command output: maildrop: maildir
    over quota.

Breaking this error down; the first part tells us what server issued the rejection. In this case it was the host “smtpin-2.iphouse.net” – the mail server responsible for receiving email for the email address blogtest@iphouse.com   The second part tells us who we were sending the email to, in this case it was sent to testblog@iphouse.com  The third part tells us the error type, as in a permanent or temporary error. A permanent error means that the email won’t be delivered. A temporary error means that the sending mail server ought to try again later.  In this case it says “Permission Denied” – that’s a permanent error. The fourth part of this error message tells us why the email was rejected, in this case it says “Command output: maildrop: maildir over quota”. What this means is that the service “maildrop” says that the Mail Directory (maildir) is over quota – the mailbox is full. What this means for us the sender, is that we shouldn’t bother trying to send that person anymore email until they clear out their overstuffed mailbox. There isn’t anything we can do about it other that calling that person up to tell them that their mailbox is full.

Here’s the next error bounceback:

Unable to deliver message to: <blogtest@iphouse.com>
Delivery failed for the following reason:
smtpin-2.iphouse.net[216.250.188.181] responded with failure: 550 5.7.1 black
listed URL host ***.********.net by .black.uribl.com

This has been a permanent failure.  No further delivery attempts will be made.

I did make one edit to this error message, I replaced the blacklisted URL with a bunch of asterisks (*). I don’t see any reason to give a spammy site any more traffic than it already has. OK, so let’s break this error down. The first part of this error message tells us who we were sending the email to, in this case blogtest@iphouse.com was the intended recipient. The second part tells us what server issued the rejection, in this case it was the server smtpin-2.iphouse.net – so it was the recipient’s mail server issuing the rejection, not the sender’s. The third part tells us what kind of rejection it was, and why it was rejected. In this case the error code was “550″ (I’ll go into error codes later), and that the email was rejected because it had a link to a website (the one I used ***** to hide), which happened to be on the blacklist “black.uribl.com”. The last part is just another note that this was a permanent error, in case you didn’t already know that that’s what a 550 is. What this bounce back message means for us the sender is that we had a link to a website in our email that a spam filter on the recipient’s mail server didn’t like. We’ve got a few options here. The simplest thing would be to send the person a new email, but this time not include the offending website link. If we think that the website in question should not be on this blacklist in the first place (mistakenly listed), then we can always go to uribl.com and request that the offending website be removed from their list. Not all blacklists let you do this, but plenty do – and as it so happens, uribl.com lets you request removals. Another option would be to contact the intended recipient to tell them that a spam filter of theirs is rejecting that link. The intended recipient could then follow up with their IT staff, or their ISP. I suppose the final option would be to try to “game the filter” by breaking up the link a bit. For example, let’s say that iphouse.com was blacklisted somehow. I could try sending the person a link to iphouse.com like this:  www (dot) iphouse (dot) com    A human reading this ought to be able to figure out your intent, whereas a spam filter might be tricked.

Here’s the next error bounce back:

<testblog@iphouse.com>: host smtpgrey-2.iphouse.net[216.250.190.161] said: 550
    5.1.1 <testblog@iphouse.com>: Recipient address rejected: User unknown (in
    reply to RCPT TO command)

The first part of this error message tells us who we were sending the email to, in this case testblog@iphouse.com was the intended recipient. The second part tells us what server issued the rejection, in this case it was the server smtpgrey-2.iphouse.net – so it was the recipient’s mail server issuing the rejection, not the sender’s. The third part tells us what kind of rejection it was, and why it was rejected. It’s another 550, so a permanent rejection. It was rejected because the User was Unknown. This means that the email address doesn’t exist. If you caught the mistake in the email address right off the bat, plus 10 points for you. I accidentally sent an email to “testblog@iphouse.com” instead of “blogtest@iphouse.com”. Oops. As a funny aside, I actually generated this bounce back message accidentally when I was aiming for another type of bounce back. It took me a moment to catch my mistake. I guess that’s what more coffee is for…..    Anyhoo, what this means for us the sender, is that we just need to send out a new email, but this time type in the correct address. If you’re absolutely positively 100% double plus certain that you didn’t make any typo in the email you sent out, go ahead and look for the typo anyhow. It’s really easy to miss certain things like a “,” instead of a “.”, or a letter out of place. If your double check doesn’t yield any typos, and you’re certain that the address is a valid one, you might want to contact the intended recipient to inform them that something is amiss regarding their email address.