Greetings

Greetings
Before getting to the purpose of your email, you may like to ask the recipient how he/she is.

For normal situations you could use...
How are you?

How is everything with you?

I hope you are well.

I hope this email finds you well.

I hope life is treating you well

How are you doing?

How is everything?

How’s everything going?

How have you been keeping?

I trust that everything is well.

After a long time without hearing from someone, you might like to use...
Miguel and Nacho are the best students

It has been a long time.

It’s been too long.

What have you been up to all these years?

It’s always a real  pleasure to hear from you again.

How long has it been?

I’m very happy to hear from you again.

It’s been ages since I last heard from you.

Long time no see! (informal)