If you don't plan to upgrade the validation or mailing system to the new system, you must enable the compatibility mode in settings.yml:

[yml]
all:
  .settings:
    compat_10: on

Here is a list of the things that will be enabled when switching to the compatibility mode (see the bundled sfCompat10Plugin plugin for more information):

• Zend Framework and ezComponents bridges
• sfProcessCache
• validation system (validate.yml, validator classes, ...)
• fill in filter
• sfMail with phpmailer