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Shivani ChaudhariShivani Chaudhari 

Error: Unable to tunnel through proxy. Proxy returns "HTTP/1.0 503 Service Unavailable

Hello Everyone,  We are making an Outbound call from Salesforce to the external system and getting below error.   Error: Unable to tunnel through proxy. Proxy returns "HTTP/1.0 503 Service Unavailable   When we try to send the same request from Postman it 
VinayVinay (Salesforce Developers) 
Hi Shivani,

Can you check with your network team to whitelist the salesforce IPs.

References:
https://help.salesforce.com/articleView?id=000321501&type=1&mode=1
https://developer.salesforce.com/forums/?id=9062I000000gAAyQAM
https://salesforce.stackexchange.com/questions/270947/system-calloutexception-unable-to-tunnel-through-proxy-proxy-returns-http-1-1
https://developer.salesforce.com/forums/?id=906F00000008ju8IAA

Hope above information was helpful.

Please mark as Best Answer so that it can help others in the future.

Thanks,
Vinay Kumar
Shivani ChaudhariShivani Chaudhari
Hi Vinay,

I m using my personal network so no need to whitelist these IPs at our end. Salesforce provided us these ips to whitelist the same at Third parties end whos service we are trying to access. but that team decline to whitelist these IP ranges due to security issues. as this Ip ranges contains total 1,22,000 thousand Domains and Sub domains.
VinayVinay (Salesforce Developers) 
Hi Shivani,

I understand your concern,  as mentioned you need to whitelist IPs and also looks like you have already logged a case and got the same response.

Try to share them the article link and probably you could explain them a bit more on salesforce platform design and issues and cause of whitelisting IP's.

Thanks,
Vinay Kumar