Dear Mrs. Woods,
Thank you for the invitation that you have sent me. I received it through my secretary and I am glad that you have chosen me to be the guest speaker for your program.
It is such a great honor for me to accept such an invitation. I understand the importance of my function and I will prepare accordingly. I am well versed with the theme which will be the topic of my speech. I have also spoken to an audience many times before and I see no problem for my incoming speaking commitment.
I shall be sending you a copy of my speech once it is prepared. Should you find anything in it that is unacceptable to you, please let me know as soon as I can so we could change it to something more appropriate. I am looking forward to the event.