邮件只要表达清楚意思就行.不像是书信高的很正式
Mr XXX(大老板）
thank you for the invitation of the party.my Client and I are all looking forward to be there.
Unfortunately,Mr YYY(客户）,my client,would not be present for the party by some(某些）reason.(be available for 也是不错的）.
As a result,i am afraid i would be there only by myself.
Meet you there.
Mr zzz(小老板）
--------------------------------------------------------------
另外很欣赏一楼的回答 真的