When using maps to navigate I always used to know where I was, more or less, at any specific time. Come satnav and I am, to all intents and purposes, lost. All I know is which way to go. It is the same with spell- and grammar-checkers.
Poor spelling is bad enough in a marketing email. If you are casual about language, what does it mean for your product? Not only that, on many text editors, neither license nor licence shows up as an error. You might feel that this allows you a free choice, but then you need to ensure that you are consistent.
Whilst alternative tends to suggest that you can pick either or any, the use of variety of spellings of the same word in an email will jar and you don’t want anything to take the readers’ attention away from your message. You should make a choice, and stick with it.
Bad grammar is infuriating. No one is suggesting first class honours degree of compliance, but if someone can’t be bothered to work out when to use me instead of I, me will let the offer go. Is A-level too much of an ask?
Some different spellings are not only correct in specific countries but for specific variations in product. Let us take whisky. If you sell in Scotland then whisky is fine, but opt across the Irish Sea and whiskey is required if you want to get by without criticism. If we move on to American English then there is a whole series of words that confuse.
If you sell mainly in Europe then, whiskey aside, you could more or less ignore the colour gray. But if you have a considerable number of American customers, or they are a major source of income, then an option is to interrogate your email marketing software and send a specific style of copy to them and another to the UK and English speakers in Europe.
Given the complexity of translating English into American English, you might consider employing an American to convert your copy. Think how you might appreciate a company from Detroit eschewing common stock for ordinary shares. Thoughtfulness is a major factor in developing customer loyalty.