How to use Gmail to send from other email addresses
1. Sign in to Gmail @ yourcompany.com first in the Incognito Window.
2. Go to Manage your Google Account.
3. Click on Security, which is a key image.
4. Click on App passwords and you will be asked to enter your password again.
5. Select Other (Custom name).
6. And name anything like gmail3 and press GENERATE
7. Copy the password in the yellow box.
8. Return to your main Gmail @ gmail.com in a normal browser. Then press the gear and then Settings
9. Click Accounts and import.
10. At Send mail as: Click at another another email address.
11. Name to understand what company we are from. And enter the email you want to send. Click Next Step.
12. Enter the password that we copied from item 7. and click Add Account.
13. It will let us enter the confirmation code sent to Your @ yourcompany.com
14. Go find that verification code in your company email.
15. Paste the verification code and press Verify.
16. That's it. You'll be able to use personal Gmail to send email on behalf of your other companies.