Indian English is closer to British English than American English, so if you specifically want it to be in Indian English but don’t know that yourself, I’d use British English.

But unless you have some special reason to believe that it’s important, I don’t think it really matters. All of the forms are pretty understandable by everyone else. I can tell that someone using British English isn’t from here in the US, but it’s not really an understandability problem. Long term, my guess is that they’ll just blend together due to international interchange anyway.

Maybe if you’re a professional journalist in the US and the publication you work at has specified American English in their style guide, they might care about your ability to specifically do that, but I can’t believe that there are many positions that would. I’ve worked with people who use British English on the job in the US.

The only specifically-Indian English word that isn’t present in other forms of English that I can think of off the top of my head is “prepone” – that is, to move to an earlier time. It’s a riff off “postpone”, to move to a later time.

Just do the needful when intimating with the recipient.

Kidding aside, you need to provide more context. Is the reason for using indian english for the recipient to understand you? In that case you don’t need to, they should understand you fine with US conventions. If your intent is to act like a local, which I don’t know why you have to, try googling sample emails and go from there.

Definitely start with “Good Sir”, that’s what I learned from years of reading voLTE requests on xda

When I think of all the shitty emails I’ve received from all over the food chain I’d say it doesn’t matter. Getting your point across is much more important than imitating a specific culture.

And seriously, badly imitating some conventions is much much worse and insulting than using a phrase that might not be familiar to your audience.

Just write it. US or UK doesn’t matter.