Question: Can any one please let me know why the sagepay throws the 5006 (Unable to redirect to Vendor's web site.), i can able to post transaction to sagepay and can able to give credit card information at sagepay's end. But, my problem is once it return back to my notification page it throws error 5006.

and i could not reach my success or fail URLs at my end.

Answer: 
Try

* <https://support.sagepay.com/forum/Topic10228-29-1.aspx>
* <https://support.sagepay.com/forum/Topic11245-21-1.aspx>

for some ideas. Just search their forum for 5006. There is plenty posts.

As a general advice: with Payment Gateways it is usually better to ask for support at their respective vendors sites.
I had this problem, and none of the suggestions here worked for me. I eventually figured out what was wrong:

1. I had to send the siteFqdns value with a trailing slash.
2. I had to send the serverNotificationUrl without the website address
prefix (ie. just the page, not the full URL)
3. I had to urlencode the serverNotificationUrl value (because it
included querystring parameters)

Only when I did all three of those things would it behave as expected.