Enhance error handling by adding error code and enhanced status code to the SendError struct. This allows for better troubleshooting and debugging by providing more detailed SMTP server responses.

Updated the TestServerPortBase from 12025 to 30025 to avoid port conflicts with other services running on the common 12025 port. This adjustment aims to ensure that the tests run reliably in diverse environments.

Implemented ErrorCode method to retrieve the error code from the server response in SendError. This method distinguishes between server-generated errors and client-generated errors, returning 0 for errors generated by the client.

Implemented new unit tests for SendError to validate the enhanced status code and error codes in various scenarios, including nil SendError cases, errors with no enhanced status code, and errors with both permanent and temporary error codes. This ensures the correctness of the error handling behavior across different conditions.

Introduce a suite of unit tests for the getErrorCode function to validate its behavior with various error types, including go-mail errors, permanent and temporary errors, wrapper errors, non-4xx/5xx errors, and non-3-digit codes.

Renamed `getErrorCode` to `errorCode` and `getEnhancedStatusCode` to `enhancedStatusCode` for consistency. Updated all references in `client.go` and `senderror.go` accordingly, improving readability and maintaining uniformity across the codebase.

Renamed `getErrorCode` function to `errorCode` for consistency. Added new tests for the `enhancedStatusCode` function to validate its behavior with various error scenarios.

Added a check for the ENHANCEDSTATUSCODES extension and included error code and enhanced status code information in SendError. This helps in providing more detailed error reporting and troubleshooting.

Previously, only the isTemp flag was considered when aggregating errors. Now, the error code and enhanced status code from the last error are also included. This ensures more comprehensive error reporting and handling.

Added support for Enhanced Status Codes (ESC) when checking the SMTP client's extensions. The SendError struct now includes the error code and enhanced status code for improved diagnostics.

Structured the initialization of SendError on connection errors to improve code readability and maintainability. This change affects the error handling in both client_120.go and client_119.go by spreading the error details across multiple lines.

Moved TestServerPortBase initialization to use an environment variable `TEST_BASEPORT` if provided. This adjustment helps in specifying custom base ports for running tests, ensuring better flexibility in different testing environments.

In the CI configuration file, the TEST_BASEPORT environment variable was added to various job scopes. This ensures consistency and allows the test base port to be set properly across different OS versions and Go versions.

Changed the SMTP test server base port and updated the corresponding environment variable name to `TEST_BASEPORT_SMTP`. This ensures consistency across the test setup and CI workflow configuration.