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 that issue and solve by updating the strYourSiteFQDN value. In my case the values was missing https and was http only. The sage will not redirect if site is https and you provide http only.