I recently had to IM a user on an external domain (John Smith). I used a valid email address and sent the message, but not one that he frequently uses. (John.Smith@ExternalGlobalDomain.com) When I tried to use an alternate domain (John.Smith@ExternalLocalDomain), Teams kept pulling up the chat for the prior domain.

Steps to reproduce.
1. Identify a user with multiple email accounts that have the same user name, but different domain names.
2. Start a chat with domain name #1
3. Start a chat with domain name #2

Result
Step 3 cannot be completed because Teams will default to using the email provided in Step 2.