Understanding Email Deliverability

March 20, 2012 Chris Leonard

Each year email service providers and recipients get better at filtering out spam.  This is good in the sense that it makes content more reliable and valuable, thus increasing the reputation of emails.  Yet an individual email does not determine the fate of future emails. Marketers find that even their quality, permission-based emails are being filtered out of subscriber’s inboxes. In order to earn a spot in a recipient’s inbox, marketers must follow deliverability standards, utilize best practices, and maintain a favorable reputation.

According to Return Path, 18% of permission-based emails in North America never reach the inbox: 5% is classified as spam and the remaining 13% are not traceable which could include various types of hard and soft bounces.  Marketers in North America have the loosest regulations compared to the much stricter SPAM laws in other parts of the world.  As of late 2012, 22% of permission based emails in Asia-Pacific, 32% in Latin America, and an astounding 51% in Africa did not make it to the inbox.

It is important to find the route of the problems before deliverability can be improved. Understanding the types of bounces is a necessary first step. The four main types of bounces are:

  • Soft Bounce:  Typically temporary issue, soft bounce occurs after 4 hours of unsuccessful delivery.  Email invalid field will not be set to true in Marketo.
  • Hard Bounce (Email address incorrect):  This indicates the email address doesn’t exist or the spam filter has reported the address as incorrect.  Email invalid field will be set to true in Marketo and the reason will contain an error message.  Marketo will then suppress future emails to this address.
  • Hard Bounce (Anti-spam Block):  Occurs when receiving email thinks the message is spam.  Marketo will then suspend all emails to this address for 24 hours but will not set the email invalid field to true.
  • Hard Bounce (Technical): has occurred somewhere between sender and recipient. There is not much marketers can do to prevent technical bounces.  Marketo will not set the email invalid to true, regardless of number of technical bounces.

In addition to these bounces, you may receive a simple mail transfer protocol (SMTP) error code that provides more information.  The following is a list of these codes and common definitions. It is important to note that the definitions are not always consistent across email servers.

Code Description
211 System status or system help reply.
214 Help message.
220 Domain service ready – ready to start TLS.
221 Domain service closing transmission channel.
250 OK – queuing for node started – requested mail action okay, completed.
251 OK – no messages waiting for node – user not local will forward to forward path.
252 OK – pending messages for node started – cannot VRFY user but will accept message and attempt delivery.
253 OK – messages pending – messages for node started.
354 Start mail input – end with..
355 Octet-offset is the transaction offset.
421 Domain service not available – closing transmission channel.
432 A password transition is needed.
450 Requested mail action not taken: mailbox unavailable – ATRN request refused.
451 Requested action aborted – local error in processing – unable to process ATRN request.
452 Requested action not taken – insufficient system storage.
453 You have no mail.
454 TLS not available due to temporary reason – encryption is required for requested authentication mechanism.
458 Unable to queue messages for node
459 Node not allowed: reason.
500  Command not recognized ‘command’ – syntax error.
501 Syntax error – no parameters allowed.
502 Command not implemented.
503 Bad sequence of commands.
504 Command parameter not implemented.
521 Machine does not accept mail.
530 Must issue a STARTTLS command first – encryption required for requested authentication mechanism.
534 Authentication mechanism is too weak.
538 Encryption required for requested authentication mechanism.
550 Requested action not taken – mailbox unavailable.
551 User not local – please try forward path.
552 Requested mail action aborted – exceeded storage allocation.
553 Requested action not taken – mailbox name not allowed.
554 Transaction failed.

Source:  authsmtp

Marketo has found that email deliverability is more heavily influenced by your reputation as a sender and less about the actual content of your emails.  According to Return Path President, George Bilbrey, more than 80% of all delivery issues arise because of a problem with your sending reputation.  This does not mean that content is not important, but it does mean that marketers need to pay attention to how their emails are being delivered. Consulting with your company’s IT department and ESP/Marketing Automation vendor on a regular basis to discuss bounce codes can be a proactive approach to increase deliverability rates.

To keep a clean reputation and ensure that quality emails don’t end up in the spam folder, marketers must encourage engagement with their audience.  Yahoo!, Gmail, Hotmail and Outlook will reroute emails to the intended recipients spam folder if users are not opening what marketers are sending out.  According to Return Path, recipients are more prone to mark an email as spam than they are to simply unsubscribe, even if you have an obvious unsubscribe link.

One of the best tips found in Marketo’s eBook (see link below) is to look through your own spam folder and consider these emails the “what not to do” example.  Once you have looked at the techniques to avoid, here are some good rules to follow:

  • Be trustworthy.  Give your potential subscribers an incentive to opt in, but don’t promise more than you can deliver.  Timely, targeted, valuable emails with catchy subjects and engaging content are good incentives for your audience.
  • Choose a reliable email marketing service provider.  Your provider should be able to handle bounce codes, feedback loops, and connection optimization.
  • Clearly brand your opt in form.  Be sure to identify the company, major products and brands or business units.

Send lists should be cleaned frequently to avoid negatively influencing deliverability rates.  Let your customers do their part by making it easier for them to update their information.  This can be done by offering change of address and frequency options at the point of unsubscribe.  In addition to offering these options, marketers should establish a time, such as twice a year, when inactive records who have not responded to anything are removed from the send list.


Utilizing best practices and the above recommendations will greatly improve deliverability but marketers must be aware and follow the rules and regulations for each geographic region (see our CAN-SPAM article). Use secure opt in techniques that tell recipients how to opt out and honor opt-out requests within 10 days.  Use subject lines and headers to intrigue your audience, not to trick them.  The subject line should accurately reflect the nature of the communication whether it be promotion or operation based. Lastly, tell your audience where you are via a physical address at the bottom of email.

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