How to properly create mocks for functions in Go when unit testing?

Hello! I started learning Go and trying to write unit tests for my project. I encountered a problem: I have a function that calls an external service (for example, sends an email or makes an HTTP request). In tests, I don't want to actually call these services, so I need to create a mock. Here's a simplified example of my code: ```go package main import ( "fmt" "net/smtp" ) type EmailSender struct{} func (e *EmailSender) SendEmail(to, subject, body string) error { // Actual email sending via SMTP err := smtp.SendMail("smtp.example.com:587", nil, "from@example.com", []string{to}, []byte(body)) if err != nil { return fmt.Errorf("failed to send email: %w", err) } return nil } func NotifyUser(sender *EmailSender, userEmail string) e