This may be unusual, but this thing just clicked in my mind. What should someone write in an email to inform their (would-be) new employer that they have posted their resignation from the current(soon would-be past) company. Also, a follow up could be a request to extend/delay the joining date(or start date, as it may be called) for some reasons? The email should be polite and is more of a request to delay the joining date. Nobody would like to be rude in the email, when a new partnership is about to begin.
In the UK it would go something like this:
Just to let you know that I've now handed in my notice, and I'll be ready to start with you guys on the nth of Octember. Really looking forward to getting started in the new role!
Kind regards, Joe Smith"
"Hi Newboss, would it be ok for me to start a week later than we originally planned? Let me know, if it's a problem we can stick to the original date."
but it sounds from the tone of your question as though you're in a location where business English is more formal than here in the UK (I'm guessing India maybe?)
If that's the case then I suggest you amend your question to say the country/culture you're in, so you can get advice that's more specific to your needs.