[SalesForce] SendID in email

Email object does have a CustomerKey attribute, as far as I know it is not visible on Email Studio page.

The only way for me to find the CustomerKey for a classic email is to make SOAP API call, RetrieveRequest for Email object, make sure "CustomerKey" in the return properties list.

This is the guide for Retrieve email via SOAP API

This can be due to various reasons:

1) The email is not sent to those contacts because there might be some ampscript logic in your email that might be looking or fetching some value from a Data Extension or real time which is not present for those contacts. Hence, it is resulting into failing of that logic and thus preventing from sending the email.

2) If you have used RaiseError() function in your email, it will prevent the email from sending if it does not meet the specified condition.

3) If you have set up a goal in your journey and the journey has reached the specified goal, the contact will exit out of the journey and the steps will be skipped, hence preventing from sending emails.

As per the description that you have provided, I strongly feel that you are fetching some value for these contacts which is not present in the DE for those contacts.

In this case please check the history of your journey to see which contact has failed to receive the email. You can then Preview and View your email for that particular contact and see if you are able to successfully Preview it for him. If not then check the value in DE that you are fetching from it.

Let me know if you catch the issue.