When sending an email, if a users' username looks like an email address, the mail is sent to that as well, instead of only the specified (actual) email.
In the example provided, an external user has a username firstname.lastname@example.org, but her actual email address is email@example.com.
The preview shows it will be sent to the ext-address as well when it shouldn't be. She will still receive the email to her actual address as well, but the ext-address will cause a bounce.
This can be circumvented by typing the full email address instead of searching for the user.
This used to work differently earlier, or at least I didn't notice this. Unfortunately I have no idea what version might have changed this.