We used AOL for a short time in the middle 1990's. Didn't like the inflexibility and the very long wait to get online as shown in the editorial cartoon below.
The AOL "walled garden" concept didn't allow for third-party apps. If you hated AOL e-mail, and many people did, you were out of luck.
When AOL said "you've got mail" - they meant AOL mail because nothing else would work.