Emails failing to be sent for months

Hi,

Emails from my website sent through Postman SMTP suddenly began failing months ago without my knowledge. I have seen many emails in my junk folder with the title ‘Mail failure – rejected by local scanning code’ and the message ‘A message that you sent was rejected by the local scanning code that checks incoming messages on this system. The following error was given. “Relaying not permitted”.’

I have tried to sign up to a new Brevo account and connect again through your wizard, but received the following error after the test email: Code: 401, Message: Unauthorized, Body: {“message”:”Your request cannot be processed. Check your email or authorized IPs page in your Brevo account.”,”code”:”unauthorized”}

After authorizing my IP address through Brevo then turning the authentication function off altogether, I received the following error after another test email: Code: 400, Message: Bad Request, Body: {“code”:”missing_parameter”,”message”:”Either of htmlContent or textContent is required”}

Can you tell me what is causing this and how to fix it?

Thanks.