Email Rejection

In most cases our MTAs will produce NDRs in response to all bounces, except where the bounce if before the email was accepted in which case the cause is returned to the originating mail server who should then return that as an NDR to the sender. 

In this article, we will discuss the common messages and their reasons. 

Sender address rejected: unverified address...

When we receive an email, the MTA will verify that the email address is valid using various techniques. RFC5321 mandates this and we enforce it, because it is just poor to have this misconfigured and generally not a professional organisation. 

Sender address rejected Domain not found

When we receive an email, the MTA verifies that the sending address is valid by checking the domain of the sender actually exists. This fundamental test helps reject spam from misconfigured servers and non-existent senders. 

Sender address rejected 550 Blacklisted...

When an email is received, the senders mail server, and the sending domain and sometimes the servers in the path are all validated against blacklists. These blacklists exist to provide real-time protection by flagging sending servers that are currently sending large volumes of spam, and when we see this indication, we either tag the email as spammy, or block the sender depending on volume and the number of blacklists currently listing that sender. 

We provide a blacklist check in our Tools section on the main website so you can see in which blacklists you are listed, and our NDR message will list the first one usually. In many cases once you've stopped the flow of spam your address will be de-listed after a while but in some cases a manual request is required to that blacklist provider. 

Relay access denied

You are trying to send from or two an email address that we do not host. This can be misconfigured DNS, or mail clients. Repeated attempts will cause your IP address to be added to the temporary ban list so it is important to fix this. 

Recipient address rejected Message rejected due to SPF fail

The Sender Policy Framework establishes which mail servers are ALLOWED to send email on behalf of a domain and this information is stored in DNS. When the MTA receives an email it verifies that the SPF information PERMITs the sending server before accepting, and if not then this is the rejection message. We often embellish it with the sending IP address and some policy information but not always. This can only be caused by DNS Misconfiguration.