If I'm sending an email to over 400K people what is the best way to make sure I'm not spammed or blacklisted? Should I send to only a fraction of the list spread out over days? Or does Marketo do this on its own?
Marketo does not do this for you. The best way to do this is to send from a warmed up respected IP Address. Most of the shared IPs on Marketo are this way. Are you using shared IPS, or do you have a dedicated IP.
Also, make sure that this is a good list. Are these people opted in? Are their addresses legit? If not you could have problems.
After I do some data cleansing it will drop quite a bit, but still should be in the 200K-300K range. Would sending over a few days spread out help?
As Jamie suggested, the best thing you can do is make sure that you are sending to valid email addresses. By removing inactive and invalid email addresses you will significantly reduce the spam trap hits. We occasionally send large batch emails and I haven't seen a different in being flagged as spam by splitting up the send.
We have sent large batches of emails a number of times.
In my experience (and excuse me for stating the obvious and something you no doubt already know) good selection of the data is essential. We are also very attentive to ensuring that certain countries are removed (we never send emails to Canada and Germany etc. unless they're properly opted in).
I would also be cautious about sending too many emails to a single company as that starts to trip spam filters too.
As with most of what we do it's the GIGO principle (Garbage In, Garbage Out).
You could spread them out if you want - it won't make a difference to the spam question but can sometimes help with the issue of Marketo slowing down because it's processing a lot of data in the background (albeit the head start feature now helps with that). I had one large data batch take 8 hours to process.