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Manage Version Settings for Visualforce Email Templates in Salesforce Classic
To aid backwards-compatibility, each Visualforce email template is saved with version settings for the specified version of Visualforce and the API. If the Visualforce email template references installed managed packages, the version settings for each managed package referenced by the Visualforce component are saved too. Saving version settings ensures that as Visualforce, the API, and the components in managed packages evolve in subsequent versions, a Visualforce email template is still bound to versions with specific, known behavior.

