It's not an uncommon case here in support - Marketo says the email was delivered, but the lead says they never got it. What happened?
Because I love analogy here's how I think of it:
Delivering email to a server is rather like delivering regular mail to a company. The post office doesn't come into the office and deliver the email to every individual desk, they deliver it to the mailroom and the clerks in the mailroom are responsible for getting the email to the desk or inbox of the appropriate person. Just as the post office considers the mail delivered when it is dropped off at the mailroom and doesn't know how the mailroom handles it from there, Marketo considers an email to be delivered when it's accepted by the server and loses visibility to what happens to it from there.
So in our virtual mailroom, the clerks have instructions to sort out spam and possibly malicious email. Depending on the instructions they have been given, they may throw some of the email away, or they may send it to the recipient but put it in the Junk/Spam folder. Why they do so is not something the post office/Marketo can see or speak to, but we can help the the recipient's IT department start to figure it out.
What Marketo can do is provide the recipient's IT department with the timestamp, return-path, sending IP, and receiving IP from our logs. This would be like the post office providing the mailroom supervisor with the time the mail was delivered, the mail carrier who made the delivery, and the mailroom clerk who received the delivery. Then they can go to the right mailroom clerk and ask, "What did you do with the email for so-and-so that we received on this date?", and since email servers log everything (which mailroom clerks usually do not) they can get the answer to this question. After the IT department determines why the emails are not being delivered, we can help them make the necessary adjustments to allow the email through, usually by altering or updating their whitelist.
There's one more behavior you can sometimes get from a very security-consious email server/mailroom. Sometimes you will look at an activity log and see that some or all of the links in the email were clicked immediately on delivery. Obviously, people need some time to receive, open, read, and click on emails, so this insta-click looks weird, but what is causing it? Well, some email security software packages have what is called link testing. The email security software itself clicks on the links in the email to check for malicious sites before delivering it into the inbox. This would be kind of like having a mailroom clerk open and check all the mail before delivering it. In the Marketo system, clicks are clicks, whether they come from a person or from an email security bot, so this can sometimes throw off your engagement statistics. Link testing in email security is not super-common, though, so it shouldn't be to much of an issue. It's just good to know what is causing this otherwise baffling behavior.
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