Alice,
These are bounces, but the telling part here is the "oob" piece of the message. This indicates that the bounce message was "Out Of Bounds", meaning that the recipient mail server first said that it would accept the email, but then afterwards bounced it. If you look at the lead records that are giving these messages, you'll see that there will be an "Email Delivered" activity right before this bounce message, which is what makes it "Out Of Bounds" since it's reporting that it both delivered it and then also that it bounced it.
This happens a lot but there isn't really any way Marketo can control it--it's entirely up to the recipient mail server. Marketo records the "delivered" or "bounced" messages back whenever they come in, but recognizes when it's gotten both that it's unusual behavior, which is why it appends that "[oob]" piece into the message.
Many times this happens when a recipient mail server takes in emails in through a main server, then dumps them over to a secondary server. The first one that takes in the email gives back the "Email Delivered" message. Then, the secondary server evaluates each email based off of spam filters and against known email addresses in their system. If they've got invalid email addresses or are filtered out as spam, they'll then be bounced, causing the bounced message after the delivered message.
If you want to track your bounces so that you don't continue to have problems, try this doc here: Maintaining a Directory of Leads Bouncing Emails
.png)