Domain DNS Checks
Check your Domain DNS setting with Google Apps Toolbox
You may get some errors like these
MAIL MX DNS checks:
error
Domain must have at least one mail server.
warning
DKIM is not set up.
warning
DMARC is not set up.
warning
MTA-STS DNS Record.
warning
There should be a valid SPF record.
warning
No Google mail exchangers found. Relayhost configuration?