When an email is sent, it passes through multiple servers before reaching its destination. If there is an issue along the way and the email cannot be delivered, a bounce message is returned to the originating server. There are two main types of bounces: hard (permanent) and soft (temporary). A soft bounce that is not resolved can indicate a more permanent issue, which is why ClickDimensions' Service Protection will exclude an email address after four bounces in three months.
The SMTP error codes that are returned in the bounce message can help you pinpoint the reason for a bounce (you can look them up at BetterBounces.net). Anything in the 400 range is usually considered a soft bounce and anything in the 500 range is considered a hard bounce (but not always). Outside of the SMTP error codes, the standards really start to diminish. Bounce messages are customizable at the email server and vary greatly. Typically, you need to consider the SMTP error code and any text that is returned to determine the cause.
The bounce message content can be seen in the Message field of an email event in ClickDimensions. The email events that indicate a bounce are "Soft bounce," "Invalid recipient" and "DNS failure."
So what can you do with all of these bounce messages? If you are experiencing high bounce rates, it's helpful to review the bounce messages to determine what's happening.
Hard bounces indicate an invalid email address. The email address most likely has been abandoned or was submitted incorrectly. These email addresses will be excluded from future sends automatically.
Some examples of messages for hard bounces:
smtp;550 5.4.1 - Recipient address rejected: Access denied
smtp;5.1.1 - Bad destination email address
smtp;554 5.1.2 - Domain relaying denied
Soft bounces provide more information and indicate an issue that can be resolved. Below are some examples of different types of soft bounces and how to resolve them.
smtp;550 5.2.1 - Suppressed using "suppressed domain" suppression list
smtp;501 5.5.2 - RCPT TO syntax error - invalid syntax
Any email that bounced using the suppressed domain list has either a typo in the domain part (think gmial.com) or is a known invalid domain that should not be used (test.com). If the email bounced with an invalid syntax, it has formatting issues. Some common syntax issues include missing the "@" sign, missing the domain extension (.com), having a space or using a comma instead of a period. These errors can be manually corrected.
SPF issue with the sending domain:
smtp;553 - SPF (Sender Policy Framework) domain authentication 553-fail
smtp;550 - Message rejected because SPF check failed
You'll want to check your SPF record (you can do that here) and, if you're a ClickDimensions customer, make updates to ensure that customers.clickdimensions.com is included and that the SPF record is passing.
Spam issues with the content or IP:
smtp;550 5.7.1 - Message rejected as spam by content filtering
Status = 5.0.0 (undefined status) | Diag = smtp;550 - Denied by policy
smtp;553 - Sorry, your IP address has been blacklisted
If you get messages like these, the IPs that you are sending from may be temporarily blacklisted (we monitor our IPs daily and work with the major blacklists to remove listings if they occur), there may be something in your email content that is triggering a spam filter or the receiving mail server has a policy in place that your mail does not meet.
Soft bounce issues on the receiver side:
Status = 4.0.0 (transient failure: all sources at per-source limit)
smtp;554 5.4.14 - Hop count exceeded - possible mail loop
There isn't anything you can do about these issues on the receiving side, unfortunately. It may be a temporary technical issue on their end that gets resolved.
Bounces are a great way for senders to identify email issues. Bounced emails should be under 5% of the total emails sent and anything over 10% is a red flag that should be investigated.
Written by Julie Turner, ClickDimensions Senior Email Deliverability Engineer