SocketLabs does not reject connections to our platform!
If for any reasons we are not accepting connections from an IP address, the SocketLabs receiving infrastructure would return an SMTP error code. You should be able to view any errors returned by SocketLabs in your mail system error logs. If your mail system does not log errors, or you do not have access to view the logs, you can always check the status of our platform here: status.socketlabs.com
Please see the below list of error codes returned by the SocketLabs platform and associated articles for an explanation of the error and how to troubleshoot the issue.
501 5.7.0 - Authentication failed. User name or password is invalid.
501 5.7.0 - Authentication required. Please authenticate with CRAM MD5 or AUTH Login
550 5.1.3 - The destination address was syntactically invalid
552 5.1.7 - The sender's address was syntactically invalid
552 5.3.4 - Message too big for system
554 5.1.0 The Sender's Address Was Not Allowed