I recently learned that people don't actually look at what the bounce back message tells them! I had no idea that the average user would just see the subject and get frustrated, irritated or give up sending the message. That black hole they call the Internet has rejected my attempts and I have no other option but to tell that person I can't email them.
Actually, those bounce back emails almost always give you a clue as to why your message was returned. It is usually just a one line error message that gives the "geek in charge" the information to know why when they get a bounce back question. Each server out there can send there own messages but there are basic reasons for the bounce back and messages usually mention one of them. Most people just don't try to look for it because it is buried in a lot of geek speak.
Here are a few common ones and what they mean to you.
Mailbox Not Found, user unknown, invalid user name - these are all messages that mean what they say. They usually mean you have something wrong in the address or the address you have really is no longer valid.
Content not allowed - this kind of response means the server on the other end is reviewing the content of the emails coming to the server and you didn't make the cut. Content rejections don't always refer to naughty nightie kinds of content. Content that can be noticed by blocking software includes things that refer to credit card numbers or social security numbers. Things that make it think you are trying to get someone to send information via email that shouldn't be sent unsecured. (You should never, ever include any social security number or credit card numbers in an email).
Once you have been blocked for this reason you have to get someone at that site to "clear" your email address. No matter how many times you try - you will be rejected. Sometimes from then on no matter what the content is in future emails.
Your address/domain has been blacklisted - Not a good feeling. Yes, it means what you think it means. You are listed as a very bad person to exchange emails with due to something making you look that way. This is very upsetting to some people and not a simple process to resolve. There are many different Blacklist services that review email traffic and try to ID the trouble servers and accounts. They do a really good job so if you end up on this list it is usually because something really is wrong, not because they are.
The last time I dealt with a client who was blacklisted the problem turned out to be a setting at their Internet provider that made them not pass a test with the ONE Blacklist company they were listed with. That one listing made emails get blocked by several large companies just because they happened to use that list. Once the ISP found the problem and made a change the name was removed within a couple of hours automatically. It did take a while to figure that one out, though.
Server not available, server busy - Sometimes, you get this message just because the server is having trouble on their end. If the server gets really busy or needs to be restarted you will get these types of messages. Those just need time to work themselves out.
Backscatter - It is very important to note there is one other bounce back that can happen which doesn't have anything to do with you. If you start getting bounce backs that are strange emails you never sent, something else is happening. You are being hit with backscatter.
Spammers will use all kinds of tricks to get past email servers and get to your inbox. In order to get around things they set up their spam to make it appear like it came from you. You never did sent it, it just looks like you did. That is why you sometimes see your own email address on spam coming in. They are tricking the servers to let them in. The Spam Filters figure out it really is spam once it gets in to the server. They reject the email and return it to who they think sent it - YOU.
Backscatter if very unsettling when it happens. Some users hit with it will get hundreds of bounce backs. Unfortunately, there isn't much that can be done until the servers clear out their messages. There is no way to know where the spammer sent the information and nothing you can do to block it. It just eventually stops.