If you invite somebody to speak at a conference, you should mention in the invitation letter the type of speech, the topic or the scope and how long the speech will be.
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Like all the business letters, the professional invitation should be written in a formal tone.
It’s rude to mention the date on which the receiver of the letter should provide his or her reply.
The first word of a complimentary close should be capitalized.
Casual words and slang as well as the light, conversational tone should be avoided in a business letter.