Requirements for SharePoint Groups when sending emails to them

I had a SharePoint designer workflow that send an email to a SharePoint group. In the History Log of the workflow, the follow message was logged:

The e-mail message cannot be sent. Make sure the outgoing e-mail settings for the server are configured correctly.

The outgoing email settings however were configure correctly (alerts and emails from other workflows were being send out).
There were actually two reasons why the emails weren’t being sent:
The settings ‘Who can view the membership of the group?’ on the group was configured as ‘Group Members’
The workflow is executed in the context of the current user. If the user has no access to view group membership, the workflow won’t be able to expand the group.
You should configure this settings as ‘Everyone’

The group had no permissions
The group should have at least ‘Read’ permission to the site. If the group has no permissions configured, you will receive the error as described above.