`unused` violation for mailer subject

[mockdeep]

Rails automatically grabs email the email subject from I18n files without needing to be explicitly referenced in code. However, i18n-tasks reports it as an unused translation:

Unused keys (1) | i18n-tasks v1.0.12
+--------+----------------------------------------------+----------------------------------------+
| Locale | Key                                          | Value                                  |
+--------+----------------------------------------------+----------------------------------------+
|   en   | campaign_mailer.verification_message.subject | Verification Code for %{campaign_name} |
+--------+----------------------------------------------+----------------------------------------+