Right, if it's being accepted by their server but not actually delivered to their inbox, or if it's going to their spam folder, your best course is check that SPF and DKIM are set up properly (you can check DKIM by looking for "dkim=pass" in the headers of a test email).
https://community.marketo.com/MarketoArticle?id=kA050000000KyrDCAS
You might also consider running some smart lists to see if you've been sending to a lot of leads who are regularly bouncing, or who have been in your database for a long time but never taken any action (never clicked or opened emails, visited pages, filled out forms). If you mail to a lot of nonresponsive leads, it will damage your domain's reputation as a sender, making your emails more likely to get blocked or sent to the spam folder.
Clearing out such leads is also a best practice to avoid hittings spam traps.