Dear Ankit,
Grammar is the way in which words are put together to form proper sentences to convey the intended meaning in communication. Certainly, it can not be an exception to mail-writing in view of the specific features of e-mails like multitude, speed, brevity, frequency of purpose etc. Grammar can again be classified into (1) Descriptive Grammar and (2) Prescriptive Grammar. Descriptive Grammar is the scrutiny of words and sentences based on the practical usage adopted by the people who speak the language. Prescriptive Grammar, on the other hand, is very much oriented with the strict compliance of the rules of Grammar as followed in authoritative sources and academic institutions. But, what is crucial is the acceptance of the fact that grammar is imperative to communicating accurately and ensuring that we are understood. Therefore, the option is inevitably dependent on the purpose of communication and the persons who are addressed. In business or official context, grammatical mistakes should not distort the purpose of communication. Long back, I read somewhere that how the wrong placement of a simple preposition "To" can disastrously change the meaning in an employment advertisement for sailors and officers in Merchant Navy as follows:
What actually mentioned in the ad was
"Note: The facility to carry wives on board to Officers only is available"
instead of " The facility to carry wives on board is available to officers only"
See, how grammar matters in communication.