Is it acceptable to use "especially" at the beginning of a sentence?

Whenever I see a sentence starting with "especially," I always get the impression that this is not grammatically correct. From example:

The high water-salt ratio will not be good for you. Especially if you are dehydrated.

I feel like the two sentences should be united with punctuation, not divided.

Am I mistaken?

• It is not acceptable in formal or academic writing. (Typically.)

  There are exceptions. @oosterwal, @tcovo, and @wooble give good examples of perfectly valid constructions starting with especially (or in wooble's case, starting with because).

  An example with because :

Because of the necessity of lowering their their baskets to the cave floor daily, the Pitifoo began to develop ingenious mechanisms to deploy the baskets with greater speed and less effort.

An example with especially:

Especially during the depression in the seventeenth century, when money was scarce in many countries, transactions frequently reverted to payments in kind. ^{The Cambridge economic history of Europe edited by Sir John Harold Clapham, Eileen Power, Michael Moïssey Postan, Edwin Ernest Rich}

Another :

Especially during the years of conflict between these powers, 264-248 B.C., the Jews must have suffered severely. ^{An Introduction to the Old Testament: Chronologically Arranged By Harlan Creelman}

• It's probably acceptable in email correspondence at work, but you have to be aware of the informal tone it gives.

  Sometimes deliberate informality can be a good thing, as it gives or creates the sense of or the illusion of a closer relationship. It is more casual as well as being more informal, and this relaxation of 'formal' rules can give the correspondence the same relaxed feel.

  'But wait,' I hear you saying, 'aren't you just deliberately and strategically making a mistake if you use it like that? '

  "Especially if you use it like that?" I ask.

  'No,' you say, 'you're just mocking me. I mean that it isn't just an ugly cousin of the correct sentence structure, which you can choose to use as you please. It's actually broken. It's a solecism."

  "Absolutely not," I insist. "Definitely not." I shake my virtual head. "The machinery of language is far more flexible than you think. The rules are beaten into your noggin so that you know how sentences function. One you know these rules and how they operate, the actual syntactic and semantic mechanisms you use to communicate your thoughts can twist, bend, and stretch to fit the shape of current circumstance."

• Among friends, it's fine, as long as one of you isn't a grammar absolutist.

• It's certainly an acceptable formation in fiction writing, as it mirrors the actual practice that occurs in spoken English.

Your instincts are on the money. The basic form of the sentence should be:

action/conclusion[,] [especially] if condition.

That said, this is one of those rules that you can break if you do so knowingly. Splitting the sentence in two before "especially" adds extra emphasis (as in @Ed Guiness's answer). As with most stylistic rule-breaking, you should do it sparingly; over-frequent emphasis loses its effect and makes you look like a twit.

You may use words like 'especially' or 'because' at the beginning of the sentence as long as you provide a supporting clause after it.

Because my dog had fleas, I had to make him sleep outside.

Especially when the weather report predicts rain, you should have an umbrella in your car.

In both of these examples you could change the order of the clauses so that 'because' and 'especially' appear in the middle of the sentence, following the comma.