I am trying to ascertain exactly the kind of person to whom you are writing.
If I did not know the addressee personally, I would keep off the subject of their health. (They may have recently been diagnosed with something dreadful, and your letter may send them into still further decline).
There isn't much wrong with the single word 'Greetings'.
But it all depends on what the message is about. If it's a cold sales attempt you obviously need to start with something punchy that will capture their attention and inspire them to take their finger off the delete button. Something like 'I promise it will take no more than 147 of your heartbeats to read this, and it could prolong your life by ten years'.
In Britain the weather is a good fall-back subject. Literally everyone comments on it daily. So if this was not my first contact I might begin:
'I trust you are well and enjoying this marvelous autumn sunshine' or 'I hope all is well with you despite the miserable weather we're having'.
If you know them really well the openings are endless, depending on how much irony you wish to employ;
'I trust you and the family are in good shape and you have come back disgustingly sun-tanned from your fortnight on the Costa Lotta'.
I trust you are in fine fettle, old boy, and up to all your usual mischief.