True, but there's no reason why that indexed primary key absolutely must be an auto-incrementing integer type.

For the particular situation of this topic, using an email address as the PK isn't a problem unless there are plans in the future to expand the system (but we can worry about that down the line, right? right?). The email address field (PK or not) will have to be indexed (unique) anyway to save repeated entries and keep the lookups speedy (e.g. when collating the addresses to send out).

All that said, there's good reason why using an integer auto-incrementing primary key is so common and it's a healthy habit to fall into.