We scrutinized the bounce messages in Marketo, as one of our teams really wanted to know how this behavior is handled. Dory is correct - Marketo only shuts off the spigot for an email address when the bounce message indicates that the email addr.... Typical response messages (these are viewable in the "Details" of an activity) : "Invalid recipient" or "Mailbox not found".
The problem is that best practice is to shut off the spigot whenever an email bounces a few times. If you don't, you look spammy and risk running afoul of the hounds - growling, howling spam filters that have aggressive names like "Barracuda" (cue the Heart song, someone). Marketo doesn't do this magically, so you really do have to program rules to take people who bounce off your mailing lists. You also need to give your sales / CRM people some feedback so that they know that a customer isn't receiving your emails - Dory's suggestion of firing back a cloned "Address Invalid" flag is a good one.
A little bit of other insight we found from analyzing our bounces:
We have all our Marketo lead activities in a data warehouse, and did textual analysis of the "details" field in order to gain some insight into what our bounce messages were (broadly) saying. We were particularly interested in seeing if there was a real difference between the "hard bounce" and "soft bounce" events that Marketo records in lead activity logs. Messages that had words like "black list", "block list", "deny list," "you are not authorized", and "denied" (to name but a few) were classified as "this place thinks we're spam." Messages that mentioned the "recipient," an "invalid character", "No user with that name", "User Unknown", and things like that were "Bad Addresses."
Then you have everything else, most of which are things like "The message failed for unspecified reasons." There is a line of thought among sysadmins that you shouldn't tell spammers that they're spamming, or that an email is invalid, or give people any help whatsoever. Some even go so far as to suggest that emails should go into a "black hole" where the email server shouldn't give any response (and others disagree). It really is an issue of having a lot of little fiefdoms, all with their own set of laws and rules, and many of which do not tell you if and when you're disobeying them.
Our text analysis found that Marketo's hard bounces were largely "you are spam", soft bounces were largely the "we won't tell you" messages, and "bad address" messages were a minority and were somewhat evenly divided between the two. Some say that those vague "we won't tell you" messages are really "we think you're spam but we're not going to tell you." Because of this, it's my opinion that you should treat all bounces - hard or soft - as equally pernicious, and equally deserving of a "stop emailing this person." This does leave the occasional person whose mailbox was full and should keep getting emails later, but we found these were few. Unless and until Marketo starts classifying these as an event of its own, your only option for automatically re-trying these after a period of time is to use the Marketo API to download the activity log, run text parsing on the "details" of the bounce events, and treat people with "bad address" language in the response message differently from all the other hard / soft bouncers.
Hope some of this helps, and that some of these resources help you get a handle on your own bounce issue. There might be a degree to which we're all dependent on each other as good email citizens, as all of our emails are coming from a pool of IP addresses owned by Marketo. Cisco's SenderBase certainly seems to be viewing these as a group (go ahead and put your Marketo instance IP address in there and see what comes up), so maybe it's possible that a strict bounce policy helps the collective reputation? I am not sure.
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