12 files declare their use of SpoolStorageInterface in Simplenews 3.x

IssueWidget.php in src/Plugin/Field/FieldWidget/IssueWidget.php

Mailer.php in src/Mail/Mailer.php

MailerInterface.php in src/Mail/MailerInterface.php

MailSettingsForm.php in src/Form/MailSettingsForm.php

NodeTabForm.php in src/Form/NodeTabForm.php

PendingSensorPlugin.php in src/Plugin/monitoring/SensorPlugin/PendingSensorPlugin.php

RecipientHandlerBase.php in src/Plugin/simplenews/RecipientHandler/RecipientHandlerBase.php

simplenews.api.php in ./simplenews.api.php

Hooks provided by the Simplenews module.

simplenews.drush.inc in ./simplenews.drush.inc

Drush commands for administer Simplenews.

SimplenewsCommands.php in src/Commands/SimplenewsCommands.php

SimplenewsSendTest.php in tests/src/Functional/SimplenewsSendTest.php

SimplenewsSourceTest.php in tests/src/Functional/SimplenewsSourceTest.php