The request is definitely going to the multisite install.
Postman SMTP - That means you do not have the same issue as the original poster. They explicitly said that the request was going to a different install.
When I see a second poster in a thread post something like
Hello there. I'm having a similar, possible same, issue.
and I already know what the first poster's issue is, I do not read through the details that the second has posted.
Try the following:
- deactivate all plugins except domain mapping on the site in question
- switch to a twenty* theme
if you still have the issue post a new thread that includes all the steps you took to set up the mapped domain.