The subject is : U.S. DoS - CEAC Confirmation (2021ASxxxxx)
And in the email it said
Dear Applicant,
Thank you for being a valued U.S. Consular Electronic Application Center (CEAC) customer. Your online Immigrant Visa and Alien Registration Application Form(DS-260) has been submitted.
I think that email is the computer-generated response to your DS-260 submission.
Same as Xarthisius's reply, the 2NL email I received in Jun 2020 was with subject: "[My DV Case#] Diversity Visa Adjustment Letter", and the body was like:
"[My name]
[My DV Case #]
Attached are the Adjustment Letter and Adjustment of Status Mail Form.
Indications on your DS-260 show that you wanted to adjust your status in the United States and indicate that you are residing in the United States and wish to process your case at the local United States Citizenship and Immigration Services (USCIS) office.
...."
The email has two attachments, both pdf files. One of them is the 2NL, and the other one is the DV Fee Paid Receipt Form.