Can someone explain what this bounce message means?

You should discuss this with Support so they can move you to another IP. Unfortunately, unless you pay for an IP, it's a problem sometimes.

550 probably means the other server said no, but not in a helpful way.

Hello Erin,

I'm a member of Marketo's Privacy Team. We handle email deliverability and compliance, and work closely with our sysadmins to maintain the mail servers.

The designation "oob" means this is an "out of band" bounce. Normal bounces happen as part of the initial email transaction - our server tries to send, and the recipient server refuses the message. An oob bounce happens when a recipient server initially accepts the message, and then later sends us a bounce saying that the message was refused. Regular email bounces arrive a standard format, but an oob bounce comes in the form of an email message sent by the receiving system. These don't follow a standard format, and it can confuse our mail servers and make it impossible for us to find the original bounce information.

It's a bit of a confusing concept, so I hope this was clear. The end result for you is that we know we received a bounce, but we are not able to learn any more about it because our automated systems cannot interpret the information that was sent to us.

As far as the Spamcop bounces you're getting, we're doing our best to reduce the impact of those issues by moving customers who cause these problems to quarantine servers and to make some changes to our internal architecture. If you send over 300k messages per month, you should look into a dedicated IP for your mailings. On the other hand, if you send significantly below this number and you are confident that you have excellent sending practices, you can contact the support team and ask to apply for our Trusted IPs. Trusted IPs are free of charge for low volume customers who meet strict requirements around data collection and sending behavior.

@autumn, if the bounce message says the folowing: 550 [internal] [oob] [Invalid Email]. Does that mean it's a true invalid email. Because I have seen that one before. 

Michelle, good catch!

Yes, I would trust that this is a true invalid email.

Have you ever sent an email to someone, and then 1-2 days later received an email response from "Mailer Daemon" or something similar that tells you that the email address you tried to contact is not valid? That mailer daemon message is an oob bounce.

Some of these are worded in such a way that our systems can interpret them, in which case you will get a message like  550 [internal] [oob] [Invalid Email]. Some of them are worded in a such a way that we cannot interpret them, in which case, you will get a message like the original poster here. Like many things in the email world, these are not completely standardized, so sometimes our systems are able to extract information from them, and sometimes they are not. We try to err on the side of caution, so if our system is absolutely certain that something is a bad address, we are much more likely to treat it as a temporary technical error. If we believe it's a bad address, we're pretty certain we're right.

Thanks for clarifying Autumn.  

This is an interesting read as we are having the same problem. We have had a return of nearly 2000 bounces that are appearing in our bounced email smartlist; so hard bounces. Looking at the reason codes, there are numerous, but one is 550 [internal] [oob] The recipient is invalid.

I am sure that some of these arent valid hard bounces but rather spam. One of our agents has been in regular email contact with a customer via outlook, but this same customer has come back via marketo as 550 [internal] [oob] The recipient is invalid.

Another one we have, under the same scenario as above, being emailed via outlook has returned via marketo with 550 5.1.1 User Unknown.

If anyone has any ideas or ways we can move forward, I would be really grateful as currently Marketo wont send to these people again but we know that some are indeed valid contacts.

Thanks :-)