package mail import "fmt" // A SendError represents the failure to transmit a Message, detailing the cause // of the failure and index of the Message within a batch. type SendError struct { // Index specifies the index of the Message within a batch. Index uint Cause error } func (err *SendError) Error() string { return fmt.Sprintf("gomail: could not send email %d: %v", err.Index+1, err.Cause) }