Dear xxx
   Hi.
   How are you? It's such a long time since I heard from you last tiime.Several mails have been sent to you sevral days ago. And I don't know whether you have received them, for the mail server is not steady these days. Pls do inform me if you receive them.
   Any problem or if you need any help, do feel free to contact me via:XXXX'

   Best wishes
   XXX
   仅供参考，不足之处,还请指正. THX.

如何发邮件问候长期没有联系的客户