Fix SmtpException.GetMessageForStatus: use distinct messages for unknown and defined-but-unmapped status codes

[Copilot]

SmtpException.GetMessageForStatus fell through default to SmtpStatusCode.CommandUnrecognized, so any unrecognized status value (e.g. (SmtpStatusCode)0 from an uninitialized _statusCode) produced the misleading message "Syntax error, command unrecognized." — implying an SMTP 500 response when none occurred.

Description

• Strings.resx: Added three new resource strings:
  • SmtpUnknownStatusCode → "Unknown SMTP status code." (for truly undefined status code values)
  • SmtpCannotVerifyUserWillAttemptDelivery → "Cannot verify user, but will attempt delivery." (for SMTP 252)
  • SmtpGeneralFailure → "General failure." (for the internal GeneralFailure = -1 sentinel)
• SmtpException.cs: Separated default from case SmtpStatusCode.CommandUnrecognized in GetMessageForStatus, and added explicit case labels for all previously unmapped defined enum members (CannotVerifyUserWillAttemptDelivery and GeneralFailure):

  // Before
  default:
  case SmtpStatusCode.CommandUnrecognized:
      return SR.SmtpCommandUnrecognized;
  
  // After
  default:
      return SR.SmtpUnknownStatusCode;
  case SmtpStatusCode.GeneralFailure:
      return SR.SmtpGeneralFailure;
  case SmtpStatusCode.CannotVerifyUserWillAttemptDelivery:
      return SR.SmtpCannotVerifyUserWillAttemptDelivery;
  case SmtpStatusCode.CommandUnrecognized:
      return SR.SmtpCommandUnrecognized;
  // ... all other defined enum members explicitly handled

• SmtpExceptionTest.cs: Added assertion confirming unknown status codes yield a message distinct from CommandUnrecognized.

Customer Impact

Exceptions thrown with undefined SmtpStatusCode values (e.g. during connection failures before any SMTP response is received) produce a confusing "command unrecognized" message, making diagnosis harder — particularly when the real cause is a transport/handshake failure unrelated to SMTP 500. All defined SmtpStatusCode enum members now map to their own descriptive messages; the default case exclusively handles truly undefined values.

Regression

No — this behavior has been present since the original .NET Framework implementation.

Testing

Existing SmtpExceptionTest suite (318 tests, 0 failures). Added assertion to TestConstructorWithStatusCodeArgument verifying that new SmtpException((SmtpStatusCode)666).Message differs from new SmtpException(SmtpStatusCode.CommandUnrecognized).Message.

Risk

Very low. Logic change in a switch statement; only affects diagnostic messages. All previously handled enum values retain their existing messages. The two previously unmapped defined enum values (CannotVerifyUserWillAttemptDelivery, GeneralFailure) now return descriptive messages instead of the misleading "command unrecognized" text.

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[Copilot]

Pull request overview

Updates System.Net.Mail’s SmtpException status-code-to-message mapping so undefined/unknown SmtpStatusCode values no longer report the misleading “command unrecognized” (SMTP 500) message.

Changes:

• Added a new localized resource string for unknown SMTP status codes.
• Updated SmtpException.GetMessageForStatus to return the new resource from the default switch case.
• Extended the functional test to assert unknown status codes don’t share the CommandUnrecognized message.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Net.Mail/src/System/Net/Mail/SmtpException.cs	Changes the default message mapping for unrecognized status codes.
src/libraries/System.Net.Mail/src/Resources/Strings.resx	Adds SmtpUnknownStatusCode resource string.
src/libraries/System.Net.Mail/tests/Functional/SmtpExceptionTest.cs	Adds assertion ensuring unknown status codes produce a distinct message.