Use this table to understand common SMTP errors and their causes.
Error | Description and information |
Service not available, closing transmission channel. The server response was: 4.3.2 Service not available | The system does not have enough permissions to connect to the host name. Validate access and permissions with your IT team. |
Port not allowed | Port 587 is used by default. The server is blocking this port. Add permissions to allow outbound email traffic through port 587. |
Mailbox unavailable. The server response was: message submission is for authorised users only! | The mailbox user (the sending email address) does not have the required permissions. Ask your IT department to review and update the mailbox permissions. |
The remote certificate is invalid according to the validation procedure | The host being used is different from the host associated with the certificate. This typically indicates a configuration mismatch with the server name. |