The Email Invalid field in Marketo is used to identify records when the lead is created and the email format is not correct, or when the email is a hard bounce that receives a 5## bounce code signaling that the email is no longer valid i.e. the email address does not exist. It’s important to know that the 500 bounce code reasons are generated by the company, so the 500 range number will match, but the reasons can vary.
To see other high-level bounce reasons, Kenneth Law wrote a good summary in the Marketo Community here. The bounce reasons also populate the Email Invalid Cause field in Marketo, which can be used to further analyze the types of hard bounces unique to your email deliverability instance.
There are a few smart campaigns I recommend be built in your operational folder in Marketo to further manage invalid emails. Before I dive into those, let’s walk through the initial Marketo/Salesforce integration as it relates to this field since it’s known to cause issues. Read the Rest on Sponge's Blog.