The name “Return Path” is technical jargon for a “Bounce”. Some would argue this phrase is incorrect because the terms have different origins and meanings around the Internet. There are over a dozen types of Return Paths and all need to be analyzed and understood for successful deliverability and whitelisting.
If you’re an email administrator, you will need to record these responses and learn the technical content behind the ISP’s so you don’t misrepresent/misinterpret a good return path from a bad one. So what are they?
Hard Bounce 550
This is the most common Return Path. Most ESP’s have a threshold of daily bounces and if you go over that, you could be suspended for spamming or asked to clean it up. Definition: The email server was temporarily unable to deliver your message to the subscriber’s email address because the subscriber’s address does not exist.
False Positive 550
When asking ISP’s for information through an SMTP handshake whether by email, DNS, SPF or for other verification purposes, False Positive 550’s can often happen if the IP is not whitelisted. Definition: User account is not there, or they turned on/off a feature that allows for non-listed users to pass through.
General Bounce
This is a common Return Path, which happens when there is a disruption in service whether a bad connection or a technical glitch. A General Bounce could still be a good email address. Monitor these and send to them at a later date. Definition: The email server could not deliver your message, but the exchange could not determine a specific reason for the bounce. The bounce message from the subscriber’s server was widespread.
Soft Bounce
This is often entangled with a General Bounce. A lot of Soft Bounces are False Positives. It’s important to monitor Soft Bounces and define them correctly because you could end up losing good emails all from a simple technical error or glitch. Definition: The email server was temporarily unable to deliver your message to the subscriber’s email address because of Internet outage, interruption and/or connection timeout issues.
Soft Bounce – DNS Failure
DNS failures do not happen too often but when they do, it could send back a False Positive. Monitoring DNS Bounces are important because the subscriber could still want to receive your messages. Like a General Bounce, monitor these and send to them at a later date. Definition: The email server is temporarily unable to deliver your message to the subscriber’s email address because of a DNS connectivity issue or incorrect configuration.
Soft Bounce – Mailbox Full
Here is where we get into the nitty gritty. Just because a mailbox is full doesn’t mean it’s a bad email address. Many novice administrators are unaware of their email settings as most defaults are set to 250 mb of storage. You should monitor these Return Paths and store them for a later date to message them. Definition: The email server is temporarily unable to deliver your message to the subscriber because the subscriber’s email box is full.
Soft Bounce – Message Size Too Large
This occurrence can happen if the recipient administrator sets a max to the email size they want to receive from the sender. This setting is common for administrators because they want to cut back on bogging down their servers with large files. Definition: The email server could not deliver your message to the subscriber because the message size is too large.
Transient Bounce
This is very common and one of the biggest hassles on both ends. Transient Bounces can last anywhere from a week to years depending on the recipient administrator settings. Unfortunately, once you have mailed to a transient, you are at their mercy, and there may not be a way to remove this until the administrator gets a clue. If you monitor your inbox, a good practice would be to contact the administrator and ask them to remove the return path. Definition: The email server temporarily cannot deliver your message, but it is still trying.
Slow Response Bounce
A Slow Response Bounce, like the Transient, can send daily messages back to you. Follow the above guide to Transient Bounce. Definition: The email server temporarily cannot deliver your message, but it is still trying.
Mail Block Relay Denied 551
Shut everything down. Regroup and analyze any and all mistakes that you are making, because if you receive a 551, you are on a spam fighter’s radar. Do you have a working unsubscribe? Do you have a physical address that is real? Where did you get your IP’s? There are hundreds of questions that can be asked in this scenario. The best thing to do is stop everything and contact the spam advisory for help. If you are not transparent with them, they could escalate the issue by sharing your domains and IP’s with others. Definition: Indicates that the subscriber’s email server is blocking email from our email server whether for spam or other reasons.
General Mail Block 550
A 550 is just like a 551 but less malignant. Follow the exact steps above. These can be resolved very quickly. Definition: Indicates that the subscriber’s email server is blocking email from our email server whether for spam or other reasons.
Mail Block Spam Detected 550
Like above. These can be resolved very quickly by changing your content using tools like Spamassassin. Definition: Indicates that the subscriber’s email server is blocking your email because the message appears to have content that looks like spam.
Mail Block Known Spammer
This technique is up to the administrator if they choose to send you this message; like a 551, but rarely sent back to the sender. Definition: Indicates that the subscriber’s email server is blocking your email because it believes you are a spammer.
Auto Reply – Out of Office / Vacation / No Longer With Company
During certain times of the year, Auto Reply responses grow. If your ESP doesn’t have the ability to track these Return Paths correctly, you can get into trouble because they could interpret these as bounces. Check with your ESP and ask them about Auto Replies and if they track them. Definition: An automatic response from the subscriber sent to your server, which could be out of office or a vacation message.
Forwarder
A Forwarder Return Path is rare but does happen. Again, it depends on the recipient’s administrator settings. Follow the same steps above as Auto Reply. Definition: Report back any disconnected user, roaming or greeting card scenario (for example, a user can instruct Gmail to forward messages to a different email address. The sender is not necessarily aware of that) called “Forwarder”.
Bounce With No Email Address (BN)
A BN can be any number of reasons. It can spark a lot of controversy if not handled immediately and/or properly. Most BN’s should be deleted. It’s not worth the headache. Definition: Determined that the message was a bounce, but it could not resolve the original email recipient email address. False positive.
Mail Block Attachment Detected (MBAD)
Some administrators block attachments because of viruses. Most smart email marketers do not send anything with attachments or have links inside with .exe endings. An MBAD could still be a good email. Consider revising your content and mail to the email address later. Definition: Indicates that the subscriber’s email server is blocking email from your email server because the email message contained an attachment.
Challenge response spam filtering
Senders should monitor their inbox for Challenge Responses. These responses require a human being to fill out a form for delivery acceptance. Definition: Spam filter that automatically sends a reply with a challenge to the sender to perform some action to assure delivery of the original message.
List Server Replies (SMTP Replies)
List Server Replies are rare. Only technical administrators that have been around for a long time still use these. They are harmless and you shouldn’t worry about these unless the administrator asks you certain questions to allow your traffic in – like a Challenge Response. Definition: A program that handles subscription requests for a mailing list and distributes new messages, newsletters, or other postings from the list’s members. (A list server should not be confused with a mail server, which handles incoming and outgoing e-mail for Internet users.)
Feedback Reports
Every sender should have feedback loops. Contact your ESP and ask about them. If you’re an administrator, contact as many large ISP’s as you can and ask them for directions on how to set them up. After you setup proper feedback reports, you will need to monitor these and follow advice from the recipient’s administrator. Definition: A mailbox provider forwards the complaints originating from their users to the sender’s organizations.
As you can see from all of the explanations above, Return Paths can definitely harm your deliverability. You need to understand what each Bounce is and how to address it properly. Most reliable ESP’s understand the above lingo and can address each Return Path with correct responses. But if you’re email marketing by yourself with an MTA, you will not get very far if you do not understand what every Return Path is and how to deal with it.
James Carner
Founder
eHygienics.com
541-771-2495
