Email get delivered initially and after a while, the email bounces stating 550 error.
This is a behavior of a particular type of Bounce called an Out of Band bounce ( OOB). The Send, Delivered and then Bounced sequence tells us that the email was initially accepted, but then due to a variety of reasons (or categories) it was rejected. This type of bounce is not uncommon, but are relatively rare.
If you look at the lead record , you'll see that there will be an "Email Delivered" activity right before this bounce message which is what makes it "Out Of Band" . There isn't really any way Marketo can control it--it's entirely up to the recipient mail server.
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 on the spam filters and against known email addresses in their system. If the secondary server gets invalid email addresses or filtered out as spam, they'll then be bounced, causing the bounced message after the delivered message.