Dear Mr.Smith:
How are you getting on?we haven't seen each other for a long time .I ve already receiced the invitation and I appreciate you that you still remmeber me. I am very glad to atternd the party ,but there are some problems ,because I have to deal with something important during those days.I,m sorry for that. I know that your birthday is coming ! I' d like to come and propose a toast with you!
Look forward to seeing you then!may you be happy in your party!
Yours,
Wang lin