In order to improve your email deliverability you should take 30 minutes and make sure you follow these steps:
- Setup MX records: https://support.google.com/a/answer/140034
- Setup SPF: https://support.google.com/a/answer/33786
- Setup DKIM: https://support.google.com/a/answer/174124
- Setup DMARC: https://support.google.com/a/answer/2466580
After following these steps, head over to the admin console of Gmail and find the 24/7 support chat. Be sure to ask support if you have correctly set up your account (MX records, SPF, DKIM, DMARC). Also ask them to confirm you what is the current sending limit on the email account.
Note: If you have a new domain or registered a separate domain for sales you should also pay Gmail $30 upfront in order to be able to send more than 500 emails a day.
Office 365 Setup: