1. Configure git.
```gitconfig
# ~/.config/git/config
[sendemail]
  confirm = auto
  smtpServer = smtp.gmail.com
  smtpServerPort = 587
  smtpEncryption = tls
  smtpUser = <gmail email address>
```

2. Configure Authentication.

I believe the simplest setup is to [create an application-specific password](https://security.google.com/settings/security/apppasswords) in Google dedicated to git.
```
Google Account -> Security -> Signing in to Google : App passwords
```

This password must be configured in git, but should be kept outside of your main gitconfig (which is likely versioned via a dotfile repo?). To do that, I use git's `[include]` directive.

```gitconfig
# ~/.config/git/config
[include]
  path = config.local # (relative or absolute path to the other config file)
```

```gitconfig
# ~/.config/git/config.local
[sendemail]
  smtpPass = <app-specific-password>
```

Alternatively, check out https://github.com/google/gmail-oauth2-tools/tree/master/go/sendgmail which supports authenticating to GMail with OAuth2.

3. Determine your revlist.

To send a single commit, just use the sha. To send a range of commits, you can use `start_sha..end_sha`. Most likely, you'll want to send the commits made to a branch that are missing in upstream. For that, you would use: `upstream/branch_name..branch_name`
You can also use relative numbers to indicate the "previous X commits". e.g. `git send-email -3`

4. Send email:
```
git send-email <revlist> --to <other@user.com>
```

5. If you receive an error like this:
```
Can't locate Net/SMTP/SSL.pm in @INC (@INC contains: ...
```
You may need to upgrade/install the Net::SMTP::SSL or IO::Socket::SSL packages:
```sh
sudo -H cpan Net::SMTP::SSL
sudo -H cpan IO::Socket::SSL
```

6. Advanced: configure multiple identites.
You may configure multiple smtp servers and switch between them with [identities](https://git-scm.com/docs/git-config#Documentation/git-config.txt-sendemailidentity).
```gitconfig
[sendemail "gmail"]
  smtpUser = jason@gmail.com
  smtpServer = smtp.gmail.com

[sendemail "outlook"]
  smtpUser = jason@outlook.com
  smtpServer = smtp.office365.com
  
[sendemail]
  identity = outlook
```