I have just received an email from the Dutch tax authorities. Subject: Uw aangifte inkomstenbelasting indienen.
U moet uw aangifte inkomstenbelasting voor het sofinummer 123456789 over het jaar 2011 indienen.
You have received an invitation from the tax authorities to file your income tax return for the year 2011. Normally these invitations are sent by post, but apparently they have your email address in their system and use that instead. This happens in case you are registered with the tax authorities as a business owner through their website for filing VAT returns.