Does your email have a proper unsubscribe link in the footer? Is your email alert setup as an operational email (and if not, does it have an unsubscribe link)? Marketo provides no warning whatsoever when emails fail to send as a result of this.
Both emails are operational, there is no unsubscribe link.
I don't think that this could cause an issue at all, as any newly registered and some existing leads are passing through the flow (receiving emails).
You mention that you were looking in a different lead database before. So, if you have multiple lead partitions and these test leads existing in one of those other partitions, they will not be added to this lead partition. It is my understanding that a lead can only exist in one partition at a time. We ran into this issue and simply deleted the leads (internal employees) from their existing partitions and then our tests for our new form worked properly.
Actually, leads will exist in two partitions: the custom-defined one (e.g., country partition) and the default partition. This is why you should always use the default lead DB when doing any lead forensics since every lead in your Marketo instance is contained here (unless you've set up a specific partition where leads are completely hidden from Marketo - like a "Duplicates" partition).
Yes, I am looking at the Default partition. I see two lead records with the same email address for some leads I used for testing. Let's put this case aside.
There is also a lead with a single record, everything is fine, but this lead does not go through the SC flow. I did not receive any alerts, the lead did not receive any emails. This is weird.
here is some more info:
I just discovered, that the program is in "Customer CRM" Partition.
The existing lead with single lead record, who did not receive any emails is present in "Sales team" partition.
Could this be a cause of our issue and Marketo simply did not process this lead in SC flow due to its initial existence in different Partition ?
However, one of the other leads (with duplicate presence in two different Lead Database partitions) is actually present in "Customer CRM" partition, but has not passed through SC flow anyway. May this duplicate presence cause a conflict ?
This sounds like our situation. If your program is in "Customer CRM" and the lead is "Sales Team", I'm thinking that is your culprit. As for the other situation, if the lead is present in the correct partition and still isn't receiving emails, the only thing I'm aware of is the setting that restricts leads from going through the flow multiple times. You may also want to check that the lead hasn't ended up on a Blocked List for some reason.
Sorry I can't be of more help, I'm pretty new at this Marketo thing. It just so happens that we use several lead partitions.
Best of luck!