Dear Mr Smith,

　　On behalf of the Student Union , I am sending this e-mail to you to invite you to act as our judge of English Speech Contest to be held in our university on May 5 .

　　To be more specific , our contest is on the topic of how to embrace tough challenges in digital era. During this competition , Ten participants will deliver their speeches with respect to this topic .

　　It is widely acknowledged that you are a top expert in this field , so we sincerely hope that you can accept our invitation . For further information , please call me at 8888888.

　　I am looking forward to your reply and arrival .