While I could RECEIVE email on multiple accounts using gmail’s business email (about $5/mo as of current date), I was unable to send email using different accounts (I was only allowed to send from ONE default account). To make this (multiple accounts) work, I had to resort to these steps.
1. Went to admin.google.com and access with the Super Admin of the account.
2. Clicked on Google Apps > Gmail > Advanced Settings
3. Unchecked the option ‘Allow per-user outbound gateways’
4. Waited 24 hours (yes – that’s how long it took). Now, when I tried to add an additional SMTP (send email) account, it let me go through!