Hello Farid,
The outcome would depend on the authentication method you are using.
If you are using Dynamics/Gmail via Oauth, then it would not be possible, as we need to authenticate the mailbox - (please find more on OAuth2 authentication for email channels here)
On the other hand, if you are using the standard authentication, it would be best to create a new channel.(more info here)
If you are keen on avoiding this, then by making the test connection, it should show the number of emails you have in that inbox.
Add the mailbox to a script, and then you can see by sending in an email.