Setting up email accounts on multi-site/domain mapping


Is there a tutorial on how to setup an email account for a sub-directory on a multi-site network?

I would like to:

1. Create a sub-directory

2. Use domain mapping to let them use their own domain.

3. Create email accounts using their domain even though they are being hosted on my main domain as they would technically always be a sub-directory.

Seems confusing and I can’t find much documentation.

Thank you kindly!