I bet you've been bitten by their switch to oAuth2. I had a customer who had regular SMTP user & password authentication switched off by MS Thursday night.

Do I have an answer on how to do oAuth2 with Chilkat? No. Using a token once you have one is fairly simple in Chilkat, but getting the token is more complicated.

Your customer should have gotten a message about turning off older style authentication - though if they use Exchange it's just a message there. My customer got the Exchange message a week ago, but of course didn't bother to tell me & now it's an emergency...

Chilkat examples:

Get a token: https://www.example-code.com/datafle...cess_token.asp

Use it: https://www.example-code.com/datafle...entication.asp