Reports can often have the same contact listed numerous times, so identifying the unique count is important to confirm the Report criteria is accurate. Having a total 9,000 unique contacts when expecting 250 is an indicator of incorrect criteria and should be investigated.
All Marketing Cloud emails are created by combining different content blocks into a layout. Check out the most commonly used blocks and those to avoid.
There are two ways to send emails through Marketing Cloud, Guided Send and Salesforce Send Emails. Guided Send has a sleeker interface but Salesforce Send Emails can be reused bringing efficiency and minimizing room for error.