(1)
Dear sir/madam: 
I'm delighted you have accepted our invitation to speak at the Conference
in [city] on [date].  (来源：英语学习门户网站EnglishCN.com)

As we agreed, you'll be speaking on the topic '' from [time] to [time].
There will be an additional minutes for questions.

Would you please tell me what kind of audio-visual equipment you'll need.
If you could let me know your specific requirements by [date], I'll have
plenty of time to make sure that the hotel provides you with what you need.

Thank you again for agreeing to speak. I look forward to hearing from you.

Sincerely yours,
[name]
[title]

(2) 

Dear sir/madam: 
Thank you for your letter of [date]. I'm glad that you are also going to
[place] next month. It would be a great pleasure to meet you at the [exhibition/trade
fair].

Our company is having a reception at [hotel] on the evening of [date] and
I would be very pleased if you could attend.

I look forward to hearing from you soon.

Yours sincerely, 
[name]
[title]

(3)

Dear sir/madam: 
[organization] would very much like to have someone from your company speak
at our conference on [topic]. 

As you may be aware, the mission of our association is to promote . Many
of our members are interested in the achievements your company has made
in .

Enclosed is our preliminary schedule for the conference which will be reviewed
in weeks. I'll call you [date] to see who from your company would be willing
to speak to us. I can assure you that we'll make everything convenient
to the speaker. 

Sincerely yours,
[name]
[title]

(4)

Dear sir/madam: 
We would like to invite you to an exclusive presentation of our new [product].
The presentation will take place at [location], at [time] on [date]. There
will also be a reception at [time]. We hope you and your colleagues will
be able to attend.

[company] is a leading producer of high-quality . As you well know, recent
technological advances have made increasingly affordable to the public.
Our new models offer superb quality and sophistication with economy, and
their new features give them distinct advantages over similar products
from other manufacturers.

We look forward to seeing you on [date]. Just call our office at [phone
number] and we will be glad to secure a place for you.

Sincerely yours,
[name]
[title]

(5)

Dear sir/madam: 
On [date], we will host an evening of celebration in honor of the retirement
of [name], President of [company]. You are cordially invited to attend
the celebration at [hotel], [location], on [date] from to p.m. 

[name] has been the President of [company] since [year]. During this period,
[company] expanded its business from to . Now it's our opportunity to thank
him for his years of exemplary leadership and wish him well for a happy
retirement. Please join us to say Good-bye to [name].

See you on [date].

Yours sincerely
[name]
[title]