Here is a simple template you can use to write an acknowledgement letter for a guest speaker.
We would like to acknowledge that [guest speaker name] was a guest speaker at our annual [event name].
He/ She covered all the essential topics in his/her speech, and it was overall very informative.
[Guest speaker name] has a great command over his/her language, which made it a great experience for the audience.
We would like to thank [speaker] for their brilliant speech and for keeping the audience gripped. We would also like to wish [speaker] luck in thier future endeavors.