Functionality not enabled.
You must send through gmail.com SMTP servers when you send as firstname.lastname@example.org.
However, this functionality is not available for your account. Please contact your domain administrator for more information.
This error usually occurs when you add a personal email address (…@gmail.com) or a Google Apps email address under your alias domain (@aliasdomain.com )
Fixing it is fairly easy but can take up to an hour for changes to take effect. If you’re the administrator for your domain, navigate to the following location:
- Login to your Google AppsAdmin Console(http://admin.google.com/ ) >> Go toApps >> Gsuite >> Gmail
- On theSetting for Gmail>> Scroll down to the bottom and clickEnd User Access
- ClickAllow per-user outbound gateways>>Allow users to send mail through an external SMTP server when configuring a “from” address hosted outside your email domains’. Tick this option and clickSave
Note: this change may take up to 24 hours to propagate to all users. You may have to wait until the next day to try setup “Send email as” again. Once you’ve done that, you can go ahead with adding an additional email the usual way, it should work now.