go-simple-mail VS viper

This library should support login auth. Does it work with Microsoft email servers?

You can however set a custom message ID, although the standard library doesn't provide an easy way to do this. go-simple-mail to the rescue:

I like go-simple-mail, it works well for my use case

Also, you can use this package https://github.com/xhit/go-simple-mail is easier than standard smtp package

MailHog is good solution for email testing, and for go smtp client here used go-simple-mail.

Once environment variables are setup we need load them into our project. For this this i will use viper

The code uses Viper to load configuration files in the application.

github.com/spf13/viper

Instead of directly accessing environment variables with os.Getenv(), integrating a configuration handler like viper might make it maintainable.

I guess most people are using https://github.com/spf13/viper but I don't know if I should read everything from

Use Viper for config file or environmental variable configuration -- it's going to save you a whole lot of time.

Cobra and Viper.

I've mostly been using https://github.com/spf13/viper of late for my go projects. It supports the standard config formats, (json, yaml, toml etc) and lets you override any value with a ENV value.