Teams free - can't add a work or school email address as an alias

Hi,

We are trying to use Teams free in our company, our work mail are:

user@mydomain.com

We have 13 different mail accounts, and no issues for 6 of them.

For the other 7 we received the following error: 

"You can't add a work or school email address as an alias to a personal Microsoft account".

We'd like to join with all 13 accounts, is it possible?

Thanks for answers