Hello/Hi + name
Dear + name
Hi there!

Informal

Dear Mr. /Mrs. + surname
Dear Sir/Madame

Formal

Opening Formula

Thanks for your letter/email.
Thanks for writing to me.
It was great to hear from you again.
I am writing to tell you about...

Informal

Thank you for your letter/email..
Many thanks for your letter/email

I am writing with reference to your letter....

Formal

Starting your letter/email

Guess what?
How are things with you? What's up? How are you? How was your holiday?
I'm sorry I haven't written for a while.

Informal

I would like to offer congratulations on ....
Let me congratulate you on....

Formal

Hope to hear from you soon.
Looking forward to seeing you/hearing from you.
I can't wait to meet you up soon.
Write back soon.

Informal

I look forward to hearing from you without delay.
I look forward to meeting you.
I hope to hear from you at your earliest convenience.

Formal

Final remarks

Best wishes
Love
Xoxo
All the best
Keep in touch.

Informal

If you start with "Dear Sir/Madam", finish with "Yours faithfully".
If you start with "Dear Mr./Mrs"..., finish with "Yours sincerely".
Regards/Best regards/King regards

Formal

Closing Formula