Where Does Dialect Stop and Grammar Begin?

Examining dialect and grammar in emails.

Here's the scenario. You work with someone who is from the south, where the spoken dialect includes words such as ya'll and ain't. You are trying to sell something to this person, so in order to appear friendly, you adopt their lingo. The usage of these dialectal words shows up in emails as well. If words such as ya'll and ain't are used in an email, is it poor grammar (in this scenario, you are someone who is a prescriptionist) or is it an extension of dialect?

My argument is that it is an extension of dialect. Even within a business setting, many people write emails as if they were talking to the recipient - more informal than "professional." And if they write as if they were holding a conversation, then it makes more sense that using dialectal words is an extension of dialect rather than poor grammar.

What do you think?