What is valid MX for sending domain?

In the most simple terms, your MX records tell the internet where your mail servers exist to receive mail. But why is this important for deliverability? Because it’s also an indicator of where you’re sending mail from. 

If your MX records are poorly configured, receiving mailboxes often look at this mishap as a characteristic of spam. If the MX records for your sending domain are incorrect, some receiving ISPs will block your mail.