The best place to look on a prospect record to help determine the reason a prospect is unmailable is the Lifecycle and Profile section. This section shows the mailability status of the prospect, which can be one of the following: Mailable, Mailable - Transactional Emails Only, Undeliverable, Unmailable, or Unsubscribed4. The mailability status is determined by various factors, such as the prospect's email opt-out preference, the do not email flag, the hard bounce or soft bounce detection, and the prospect's location in the recycle bin5. The Lifecycle and Profile section also shows the email address, the email preference center, and the email opt-out date of the prospect, which can provide more information about the prospect's mailability