Hi Jeffrey, do you know if you've whitelisted Marketo IP addresses on your company server?
Often, emails going to your company server are more likely to be blocked because the From address matches your company domain, yet your server can tell their coming from an outside source, which looks more suspicious to security software (it looks similar to phishing - someone pretending to be from your company in order to steal information).
I recommend doublechecking with your IT folks that Marketo's sender IP ranges are whitelisted:
To add on to Grant's comment, I would try sending a test email to a non-company email address, like a gmail, so you can identify the source of problem.
Looks like you were most likely both right. After reading Grant's response I did send to outside servers and the flow succeeded. Still waiting for confirmation from IT, but looks like the whitelist was the issue. Thanks!