how to used external secured email gateway with

We have on premise installation with external email server setup

Earlier we were using unsecured email gateway and emails were working fine.

Now our IT has changed that email gateway to a secured email gateway. We have changed postfix configuration and provided relay server, username and password but getting following error.

2/0/0/0, dsn=4.7.0, status=deferred (delivery temporarily suspended: SASL authentication failed; cannot authenticate to server[]: no mechanism available)

Any idea what could be wrong here ? Please help as this has blocked our chatbot operations.