Chat with external contacts - possible without them being in a Team?

Chris Hughes    Its possible and you need to check if External Access is enabled.  .

https://docs.microsoft.com/en-us/microsoftteams/manage-external-access

Use external access when:

• You have users in different domains who need to collaborate. For example, Rob@contoso.com and Ann@northwindtraders.com are working on a project together along with some others in the contoso.com and northwindtraders.com domains.

• You want the people in your organization to use Teams to contact people in specific businesses outside of your organization.

• You want anyone else in the world who uses Teams to be able to find and contact you, using your email address.

Chris Hughes Hello, external access is turned on by default in Teams, which means that your organization can communicate with all external domains. But this can be configured in different ways (short answer is yes).

Manage external access in Microsoft Teams

https://docs.microsoft.com/en-us/microsoftteams/manage-external-access 

Let me highlight these as well.

Compare external and guest access

https://docs.microsoft.com/en-us/microsoftteams/communicate-with-users-from-other-organizations#compare-external-and-guest-access

Communicate with users from other organizations in Microsoft Teams

https://docs.microsoft.com/en-us/microsoftteams/communicate-with-users-from-other-organizations