What does it mean for a constituent to be in or out of the "email opt-in universe?" For instance, if someone is not marked with the email opt-in universe, and an email is sent to "all records with email addresses," does that person still get an email?
== response by Blue Utopia ==
The answer to the question above is 'yes'.
The list "All Records with an Email" is exactly that -- every record in your system that has an email address.
By contrast, the list, "Email Opt-In Universe" includes the an additional layer (code = email opt-in) which allows you to pull just the records that have an email AND have been specifically marked to get your emails.
The act of including people on this default email list is done in a variety of ways -- flagged at time of import, a checkbox when adding a record, automatically via the settings in sign-up pages, etc. The results is that 'most' records are likely (or should be) added to your Email Opt-in list by default.
We generally recommended that you use 'Opt-In Universe' to contain your email audience. Here are some things to consider:
Maybe you don't want to put everyone on your email list. For example, you might want to leave off Press contacts or high-dollar donors from your main email list. This approach helps you to communicate to your broad audience but still keep control of leaving off target groups for more custom outreach.