I imported a list and used that list in a campaign and noticed it only sent to half the people. When I took a deep dive into the people who didn't get sent the email I realized they were all DNS in SF (which makes sense). When I looked them up in Marketo some were in the lead database and some weren't. Why would this be? Also I noticed no type of DNS was checked off under the lead info tab in Mareketo. When does Marketo mark these off? I noticed for one lead, in SF under their profile>Marketo > MKTO invalid email was checked off but in Marketo the lead didn't have such box checked off. Why would this be? When does Marketo determine if an email is DNS and does it update in both the lead database and in salesforce? If the DNS status changes does Marketo update? And will that lead get future emails? And past emails it qualified for?
Marketo syncs with SFDC every 5 minutes. Every record that has been updated on one end of the sync will be pushed to other end.
Every record created in SFDC during the past 5 minutes will be pushed to Marketo, but the reverse is not true. new records in Marketo are pushed to SFDC only if the run through a "sync with SFDC" or "Add to SFDC campaign" flow step or if they are added as members to a Marketo program that is sync'ed with an SFDC Campaign.
It may happen that some records in SFDC have been blocked from sync to Marketo, which would explain why they would not sync.
Marketo standard Unsubscribe field is mapped with SFDC "Email Opt-Out' field. There is no SFDC standard field names DNS. Either it's a custom field or it has been given a different label in SFDC. If your DNS field in in fact the email opt-out field, no problem, it will sync. But if it's a custom field, things are much more complex: it would take some specific campaigns in Marketo or workflow in SFDC to guarantee the sync.
To make things even more complex, some other fields, in addition to Unsubscribe, may explain why Marketo would not sen emails: email invalid, blacklist, marketing suspended, ...
At the end of the day, it's a very complex issue and we are missing too many info to be able to point you to a root cause.