Here are some details about how Marketo emails are sent and tracked.
With those changes complete, Marketo will attempt to deliver your email to the recipient.
Once the email is sent from Marketo's servers, a Send Email event is added to the lead's activity log. If the mail was successfully delivered, Marketo logs an Email Delivered event.
Sometimes an email can't be delivered. Marketo will place these email bounces into two categories:
Sometimes an email isn't deliverable immediately, but might be okay later - for example, if a recipient's mailbox is full. This is a soft bounce and is logged as an Email Bounced Soft event. Marketo attempts to deliver the message up to 3 times, trying each Mail Exchange (MX) server. With each attempt, the timeout value is increased.
Sometimes an email cannot be delivered no matter what, for example. the email address is incorrect. This is a hard bounce and is logged as an Email Bounced event. Marketo sets the lead's Email Invalid field to true so that the lead will not be emailed again. The Email Invalid Cause is populated with a reason returned by the mail server.
Emails may be blocked for a number of reasons, including, spam complaints, blacklisting, or because some aspect of the email content has triggered a spam filter. When an email is blocked it is logged as an Email Hard Bounced event. Marketo sets the lead's Email Suspended flag for 24 hours during which they cannot receive email.
Sometimes leads will mark your emails as spam or junk mail. When that happens, Marketo is notified of this event by the lead's email provider, and Marketo will automatically unsubscribe the lead from the emails by setting the Unsubscribed flag to true.
When a recipient views an email, their email client retrieves the invisible image from Marketo's servers. This triggers an Open Email event, no matter how long or short the view was. An Open Email event also occurs when a lead clicks the "View as web page" link in an email.
Only one email open event is tracked regardless of the number of times a lead views the email. This happens because email clients make the open event unreliable. For example, a lead could open the email multiple times just by browsing through their inbox via the preview pane reading the entire message content without clicking/opening the actual email, or the lead's email browser may block images which prevents Marketo from tracking the open events.
Due to Marketo's distributed architecture, email open events may be delayed a couple of minutes between when the event occurs and when they appear in the lead's activity log.
The links in an email are wrapped by Marketo with special tracking code. When a recipient clicks on one of those links, the Marketo servers are informed about the click, and a Click Email event is logged to the recipient's activity log.
A person who clicks one of these links also gets cookied by Marketo; this makes them a known lead and causes subsequent web activity (on Munchkin-enabled pages) to appear in their activity log.
When the "View as web page" option is used for email, the views and clicks on those pages work just like views and clicks on the actual email in the lead's inbox. For example, any click on a link on the email web page registers in the lead's activity log just like the lead clicked it in an email client.