Hello Rich,
You're right. We mark leads as email invalid when they come back with a bounce that tells us it's a bad address. This is a permanent status (unless manually changed). Leads set email invalid will not be sent additional messages - they're part of the "blocked from mailing" number shown in your campaign schedule tab. To repeat, even though these lead records remain in your database, we will not actually send them an email, even when they qualify for a campaign.
Some customers choose to delete email invalid leads, while others keep them in the system for historical reporting, research, or other purposes. What's right for you will depend on your specific business practices.
Leads who get set email suspended are those that bounce due to a temporary antispam block. These are usually good addresses, but they are temporarily unavailable. They get suspended for 24 hours after the email bounce, and then they become available for mailing again after that time. (A note - the email suspended field still stays set to true even at the end of this time - check the "email suspended at" field for a timestamp. 24 hours after that time, the lead will be available for mailing.)
We recommend that you keep leads who get set email suspended. While some antispam blocks can be long term, most of them are resolved within a matter of hours or days.