Sent message may be bouncing back as undeliverable for a number of reasons which include:
- Recipient's mailbox is full
- Recipient's e-mail address is mistyped or the e-mail account does not exist
- Recipient's mail server is not available or down
- Recipient's e-mail account is set to exclusive mode and can only receive message from people in their Address Book
- User's e-mail address is blocked by the recipient