Teams can not chat with email user

Hi raphir 

It also depends on the multiple factors:-

1. What is the Address which the Org you are trying to use is using. It could be a possibility that the other org is having different SIP and SMTP Address. They might be using user1@abc.com as their Email Address and user.1@abc.com as their SIP address.

2. It also depends what kind of external communications they have enabled for communications with the external organizations. They can have closed communications with only selected domain so you might need to ask them to add your domain to their list.

3. What is the Services external domain are using are they using Microsoft Teams or Skype for Business On-prem or Skype for Business Online? 

Hope you would be able to find the answer to your queries in one of the above questions.

With Regards,

Satish U