As you may know, in HubSpot you can create lists whose criteria is based on email events, for example Email | was sent | [Name of sent email] or Email | received | [Name of sent email].
After creating such lists you may find a contact, which you believe does not fit the criteria, but was pulled into the list nonetheless. This is because email list segmentation reviews all historical email addresses on all contact records, and not just the current addresses.
For example, let's say you have set up a list with the criteria Email | was sent | Email A, and among Email A's recipient list there the email address firstname.lastname@example.org.
With that said, you have a contact with the email address email@example.com, who has never been sent an email, but for some reason it is getting pulled into the list. By looking at the contact's history, you noticed that at some point this contact had the email address firstname.lastname@example.org - This is why the contact was pulled into the list. Email list segmentation reviews all historical email addresses on all contact records and if a contact has ever had an email address corresponding to a sent email's recipient list, such contact will get pulled into the list.