I realise this is a super old discussion: but you are worrying too much about rules. Punctuation is there to serve us, not be our master. Use as you wish, for effect, for clarity, however you like. There really is no "correct" with most of this stuff. There's better, there's worse, and there's different, but beyond that it's your choice and your power as the writer to bend this stuff to your will. The fact is that written prose doesn't actually match very well how we think or how we speak. (Record a conversation some time and and then transcribe it - you'll be surprised). As such, writing an email to someone is quite an artificial task - it's a kind of synthesis. Really the only purpose is to communicate your thoughts and if you have done that in a broadly appropriate way (ie not text speak!) then you're good.