We would like to reach out to past members via email. This is going to span quite some time and there is a high likelihood that many of these people have old email addresses that are no longer valid (business email). What happens if we send an email to a few thousand (maybe even 5K) people and they get hard bounced? Do we get blacklisted somehow? I just want to make sure the project is worth it. Thanks!
You won't get blacklisted unless your list has dirty data and contains email address that are known spam traps. This usually pops up with purchased lists.
If these folks have previously been sent emails (seems like you referred to them as past memebers?) through your Marketo instance you should be good to go. Even if they haven't been sent through Marketo before, as long as you know it's not a purchased list with dirty data, you shouldn't run astray of Marketo policies that would get you blacklisted.
If you're overly concerned about this because it's inherited data you're not 100% sure about, you can break this list into small batches to see if you get dinged. I'd contact support on this option b/c you may want to let them know what you're doing.
Coupled with Jimmy's comments, some spam monitoring companies acquire old email addresses that were once real and keep them active as spam-traps - so that is a risk when emailing inactive data. The purchased list spam traps are a lot more popular though.
Are you on a dedicated sender IP at Marketo at the moment? Activities like this can damage your sender reputation if so, which will result in reduced inbox placement - but this is more of a risk with dedicated IPs.
You are aiming right with smaller batches though so I would pursue that if you move forward. Best of luck with your campaign!