Mandril/webhook post issue

Folks.

 

I am facing issue with Post which using user agent as Mandril/webhook were if the payload is more than 1mb its failing to upload to IIS server over F5 LTM , if the payload 100 to 500 kb upload is successful. connectivity as below client (webhook) - F5 VIP(SSL offloading) - Backend server IIS port 80 what we observed is during troubleshoot that payload is reaching the F5 VS but when sending from LTM self ip to back end server its sending only 16kb and IIS logs reported error as 500 for not getting complete post data.many traces taken and troubleshooting (played around with TCP & HTTP profiles)done but not able to identify what is causing the issue. due to production environment i cannot terminate the ssl to directly backend server to test.

 

appreciate any help to narrow down the issue.