3rd party email server

I have been using groundskeeper Pro software to send out email bills to my clients who have balances.

This month October my billing no longer works.  I have tried setting up the email server settings in Groundskeeper pro again but to no avail.  I contacted Groundskeeper pro support and they said I need to use a mail server that is compatible with a 3rd party bulk emailing engine.

Did something change in outlook that no longer allows me to use a 3rd party email engine?

I was setting it up as server smtp-mail.outlook.com  port 587  but this no longer works.

Is there a work around for this?

Mark