A large percentage of the internal distribution groups are used by "non-person" automated systems for mail distribution. By default, when groups are provisioned using the tool, the "RequireSenderAuthenticationEnabled" attribute is set to a default value of $True.
You may need to override the default behavior and set this attribute to $False.
Use PowerShell or a script after the groups are created.
Enhancement request number TFS486173 has been submitted to Development for consideration in a future release of Migrator for Notes to Exchange.