You’ll need to send the messages using the email provider’s SMTP servers, so log into gmail and go to settings. Her is the link https://mail.google.com/mail/#settings/accounts
Then go to the Accounts and Import Tab
Then click edit info next to the address you want to change the “Sent from” smtp server
Then click button
Then select Send through addressOfyourSMTP.com SMTP servers (recommended for professional domains – Learn more)
After this you should be good to go!