Here's the scenario:
1. We delete a Lead in Marketo because the email is invalid. We do not want to send additional emails to this address, nor do we want to pay to store this record in our Marketo database. But we do not delete the Lead in Salesforce, because it might be a legitimate person with a bad email address.
2. In Salesforce, if a sales person updates the email address for the Lead, that change does not sync back to Marketo, because the Lead no longer exists in Marketo.
Has anyone found a way around this? Or is there a better way to deal with the issue? Only a very small percentage of sales people update the record with a new/correct email address, FYI, which is why we don't want to keep the records in Marketo.
As long as the sync user in your SFDC instance can still see the record, then when it gets updated it should sync back down to Marketo as a new record. Deleting the lead in Marketo doesn't permanently ban that record from ever syncing back down, and in fact, any update to the record in Salesforce should sync it back to Marketo during the next sync cycle.
If you're not seeing this happen, first check to be sure you don't have some kind of permissions/rule blocking this lead from the sync user's visibility. Also check to be sure you don't have a custom sync filter with a field like Sync To Marketo that needs to be checked in order for the record to sync down. And finally, if you're still not seeing the record come done after verifying that the sync user can see it, please open a case with support.
Your assumption on number 2 is incorrect. When a change is made to the record, it will sync back as a new record into Marketo.