The mailbox server in the 3CX server is very important for the 3CX system. The mailbox server is used to send various notification emails, such as sending welcome emails to extension users and sending various types of event emails to designated extensions. Our free version of 3CX currently uses an SMTP server provided by 3CX. The commercial version license can use Gmail, office365, outlook or a custom SMTP server. When we test that the mail server is normal, it will display Mail sent in green font and background. If it is not normal, it will be displayed in red font for a variety of reasons. In this document, we introduce the possible reasons and solutions for the 5.7.0 Denied error in the 3CX mailbox server test. This problem occurs mainly on servers that use 3CX SMTP mailbox servers.

The reason 5.7.0 Denied error appears

This error usually indicates that the IP address bound to your server FQDN does not match the IP requested by the email. For example, your current domain name is test.3cx.cn and the bound IP address is You can to to your 3CX portal to view the domain name and IP address of your license binding, or to ping your domain name to view the current IP address. Your current 3CX server’s current public IP is not, but, then the mail request test sent from this IP will have a 5.7.0 Denied error because the request is not from 113.112. 111.110 this IP address.

Reason for IP address mismatch

  • Some users install 3CX equipment using multiple network cards, one network card is used for 3CX, and another network card may be used to connect to other network equipment, such as the SIP line provided by the operator. In this way, the public IP used in the request sent during the mailbox server test may be the IP of another network card instead of the IP address bound by 3CX.
  • Some customers’ 3CX server does not have a fixed public network IP address, which also causes this problem. When the customer’s 3CX public network is changed, the IP bound to the FQDN is not immediately updated, but it takes some time to update the DNS resolution. Then the IP address used by the mailbox server during this time is not consistent with the IP address bound to the 3CX FQDN, and a 5.7.0 Denied error may occur.
  • Some customers install the 3CX server’s public network IP address type selected by static 3CX, which is static, and the company changed the public network IP address during the subsequent use, but did not update the public network IP in the 3CX control panel settings Address, which will also cause IP address mismatch, resulting in a 5.7.0 Denied error.


For users using multiple network cards, we recommend that customers set priorities for network card devices to ensure that the data sent by the 3CX server is sent through the IP bound to the 3CX domain name. We do not recommend deploying commercial 3CX in an environment without a static public network IP. Of course, if the customer has no need for email notification and only uses it on the internal network, then there is no problem.

In addition, when a 5.7.0 Denied error occurs, the customer can modify the static public network IP in the 3CX control panel version settings-network-external network IP configuration to a dynamic public network IP and restart all 3CX services. Then try refreshing the activation license and test SMTP again.

If the commercial version of 3CX is licensed, customers can use other centralized mailbox servers or use custom SMTP mailbox servers.