In some cases sending e-mail would not be successful and no exception will be logged, as sender_from configuration is required in some swiftmailer configuration, but not provided by default and must be set explicitly:

swiftmailer:
    sender_address: 'no-reply@example.com'

Steps to reproduce
1. Clean installation of eZ Platform 2.x
2. Configure Swiftmailer with Mailjet (or any other provider which requires the sender to be set)
3. Create a new form with e-mail notification.
4. Fill the form.

Result
No mail is sent.