I would not recommend saying 'Long time no see' to clients as it is a slang term used to greet friends or colleagues. It is better to email your clients something like this; Dear Mr Smith, As I haven't heard from you for a while, I would just like to inform you about the new advances our company has made/ new products that have been developed etc. [ Add more here......] Please contact me if you would like more information. Looking forward to hearing from you soon, Yours sincerely, Miss Chu Marketing Manager Phone: zzzzzzz Fax: yyyyyyyyyy

we usually say : "long time no see, how are things going?" or simply "Long time no see, how are you?" I think asking someone if "everything goes well" is perfectly fine if you knew that your client (especially a regular client) was not in good shape or was not having a good time prior your next meeting / e-mail conversation. be careful because your client might misinterpret your greetings, thinking that you're being too speculative. :)