Upon sending your emails, you must use a send domain that is verified with sparkpost.
Let me explain how this works.
If you have added a send domain in sparkpost of ABC.com. In you "from Email' address, you MUST put an address that is from @ABC.com as this is the only email domain that is verified to send emails using sparkpost.
To send from other domains, you will need to add them by logging into sparkpost and goto to accounts -> Sending domains. Then you must verify the domain using DKIM as per our setup documentation.
You cannot setup @gmail or shared email domains using sparkpost, having said that we advise strongly against using them for your Marketing Endeavours where posible.