As of the date of this note you can simply use the Gmail SMTP server, as long as you use Google two-step authentication.

So just for completeness, the full steps:

1. Gmail settings, Accounts and Import tab.
2. Add another email address you own
3. Type name and email address to be added.
4. For SMTP Server, put smtp.gmail.com
5. For Username, your full Gmail address including @gmail.com
6. For password, provide an App Password generated in Google Accounts at https://security.google.com/settings/security/apppasswords
7. Leave Secured connection using TLS selected as is.
8. Add Account

References

• Stackoverflow