Introduce two new tests in senderror_test.go: TestSendError_MessageID and TestSendError_Msg. These tests validate the behavior of the MessageID and Msg methods of the SendError type, ensuring correct handling of message ID and sender information.
This commit introduces a new test case to verify the behavior of the IsTemp method when called on a nil SendError instance. It ensures that the method returns false as expected, improving test coverage for edge cases.
Did a complete overhaul of the senderror.go.
- the list of `errors.New()` has been replaced with constant itoa error reasons as `SendErrReason`. Instead, the `Error()` method now reports the corresponding error message based on the reason.
- The `SendError` received a `Is()` method so that we can use `errors.Is()` for very specific error checking. I.e. we can check for `&SendErrors{Reason: ErrSMTPMailFrom, isTemp: true}`. This provides much more flexibility in the error checking capabilities
- A `isTemp` field has been added to the `SendError` type, indicating whether the received error is temporary and can be retried or not. Accordingly, the `*Msg` now has a `SendErrorIsTemp()` method indicating the same. The decision is based on the first 3 characters returned from the SMTP server. If the error code is within the 4xx range, the error is seen as temporary
- A test for the SendError type has been added