In case anyone is still looking for the categories defintioins - Here is the official response from Marketo
Category 1: Spam.
A receiving email server will return a Category 1 Hard Bounce when it regards the email as spam. Many times, this is a temporary bounce and sending to this server will be possible after 24 hours. When Marketo receives a Category 1 Hard Bounce, it will mark the lead as "Email Suspended". For 24 hours after this marking, Marketo will not send anymore emails to this lead. After 24 hours, sending can continue to this lead without any input from you.
Category 2: Does not Exist
A receiving email server will return a Category 2 Hard Bounce when the email address being sent to does not exist or is invalid. When Marketo receives this response, it will mark this lead as Email Invalid. No future emails will be sent to this lead unless this value is manually set back to false.
Category 3: Technical Issue
A receiving server will return Category 3 when they are experiencing some technical issues. No action is taken on our side for these bounces.