Looking on the internet deeply has found these results:

For your unexpectedly prompt reply is the most popular phrase on the web.

For your unexpected prompt reply 11 results on the web	For your unexpectedly prompt reply 14 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
up to Christmas, I would appreciate a prompt reply to the following questions: 1. As 1 July is imminent, a prompt reply would be much appreciated. Secondly, Parliament's proposal on the matter received a prompt reply. I would like to thank Baroness Ashton for her prompt reply and her reassurance two weeks ago that the matter will be treated on the highest possible level in the EU. Another of your unexpected guests? As the issue of toy safety is being debated critically above all in the run And she's promised a prompt reply.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

For your unexpected prompt reply

11 results on the web

For your unexpectedly prompt reply

14 results on the web

More popular!

up to Christmas, I would appreciate a prompt reply to the following questions: 1.
As 1 July is imminent, a prompt reply would be much appreciated.
Secondly, Parliament's proposal on the matter received a prompt reply.
I would like to thank Baroness Ashton for her prompt reply and her reassurance two weeks ago that the matter will be treated on the highest possible level in the EU.
Another of your unexpected guests?
As the issue of toy safety is being debated critically above all in the run
And she's promised a prompt reply.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.