Introduced NTLMv2 authentication mechanism with tests in smtp package. Utilized the go-ntlmssp library to handle negotiation and response processing for NTLMv2 protocol.
Upgraded golang.org/x/crypto to v0.28.0 and golang.org/x/text to v0.19.0. These updates improve security and compatibility with recent changes in Go modules. Ensure to run `go mod tidy` to clean up any unused dependencies.
Added `golang.org/x/crypto v0.27.0` and `golang.org/x/text v0.18.0` to go.mod. Updated go.sum to reflect these changes for proper dependency management.