How should I begin an official e-mail if I don't know who I am writing to? I mean, normally I would write "Dear x", but when I have to send an e-mail to an institution, what should I write?
When writing a formal email without knowing the name of the receiver, I would tend to fall back on standard letter writing style, especially for job applications and the like:
Or if the email doesn't need such an air of formality (for less important things), I'd write a simple:
Hello [company name],
Email and paper letters are completely different things, and should be composed differently.
For example, all emails come with automatically composed headers that contain (or at least are supposed to contain) all the contact information required for the recipient to discern who the message is from and exactly who they thought they were sending it to. Thus salutations, while vital for formal letter-writing, are annoying and redundant in an email. They should almost never be used.
The tone of an email generally needs to be far less formal than a written letter would be. There are historical reasons for this, but the main modern consideration is the deluge of spam everyone now receives. Messages that are clearly meant for mass or unknown recipients are unlikely to make it through spam filters, whether human or computerized.
Thus if you don't have a prior ongoing relationship with the recipient(s) you do need in the very first line to establish who you really are and justify why you are contacting this person. For example, my first line in my first email to a tech support contact I was given by a sales person would be something along the lines of:
I'm having some trouble with Product_X, and Salesguy_Y suggested I contact you.