Connection not possible to Office 365 in GPOADmin. Proxy is setup and Firewall ports open. Browser connection to Microsoft Login is working. Application in Entra ID is set up and Mailbox has permissions to use it. The certificate is setup as well in Entra ID app and GPOAdmin.
Currently, GPOADmin does not support setting proxies within the product.
We have logged an enhancement to have HTTP proxy support added to GPOADmin. The enhancement to be considered for inclusion in a future version of GPOADmin is 539195
ER 539195: Add support for accessing InTune / Office 365 through an HTTP proxy server
