How to handle blocked emails

When campaigning using Custobar, you might find that your total recipients number is smaller than the segment you created. Or a customer might reach out asking why they stopped receiving your newsletters. The reason is often a blocked email status. Let's have a closer look at this topic.

What is a blocked email

There are two types of events that can prevent a valid, existing email address from receiving an email. Hard bounces and soft bounces.

Soft bounces mean the email cannot be currently delivered for a number of reasons.

These reasons range widely, but here are some examples: Mailbox is full, receiving server is down or offline, DMARC requirements for authentication are not met, anti-spam or anti-virus filters are triggered, or the domain name does not exist. Soft bounces are usually a temporary issue, but can indicate a deeper problem. A full mailbox, for example, can show that the address in question is really old and not in use anymore. A domain name that does not exist might mean the address, while looking valid, is actually not in use at all. While we can try to reach to that user again with another sending, soft bounces are usually a good sign that something is wrong, and being too insisting might actually harm your sender reputation. This is why at Custobar, we set the rule that too many soft bounces will be treated as a hard bounce and block the email.

Hard bounces are permanent issues and a hard bounce response will have Custobar automatically block the email address from future sending until manually fixed.

Hard bounces are caused by two cases. The best case: the address doesn't exist. This is easy to solve and is usually pretty harmless. The worst case: the receiving server is categorically blocking your email. This is where your sender reputation is at stake: the receiving server has flagged your message as too suspicious to deliver or your sender IP is listed among suspects of spam content, your sender authentication is failing to meet basic requirements, so on and so forth. Trying to resent an email to a hard bouncing address is very risky for your reputation as a whole, which is why only manually fixing the block will allow you to retry. Otherwise, the hard bounced address is blocked and will be skipped automatically in future sending.

Email block types

You can check blocked customers and segment them based on the status code of their blocked email. The field blocked_email can have different values. As a rule of thumb, anything above 0 means the mail is blocked. 0 or a null value mean the address is not blocked. Here are the main values and their respective meaning:

  • 2 Spam report
  • 3 Too many (4-5) soft bounces
  • 4 Hard bounce
  • 5 Invalid
  • 0 Indicates no block

How to handle blocked emails

If a customer's email is blocked, the easiest way to reset the block is to change the email address to a valid, in-use one. Simply ask the customer what email address they are using nowadays and edit their profile.

If the customer is positive that the blocked address is valid and currently in use, you might simply release the block by editing the address and then reverting to its original form. So could be switched to and then back to

Please note: Doing so will release the block but it will not fix the underlying issue. The next sending could just as well result in a new block, and your sender reputation will take twice the hit from the two consecutive blocks. Doing this every now and then is fairly safe but should not replace proper contacts management.

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