Some issues occur on the new setup when trying to send mail from FMS.
Quote removed at author’s request.
A general comment, not specific to O365 :
Over the next few months and years, there has been and will be a further a general tightening up of email encryption, authentication and security generally on all world email systems. This means that systems which work fine now, are quite likely to stop working with little or no notice, requiring either further software updates at the SMTP sending end, further updates to the DNS of the domain name upon which the From: address is based, or possibly even requiring you to switch to a different provider. December 31st 2019 will be a milestone in particular.
Those using a Filemaker Plugin are likely to fare better than those not using a Filemaker Plugin, as you have a developer working full time on keeping your SMTP compliant with the latest standards.
Bear in mind that Microsoft, Google et al provide their SMTP service primarily for use with a desktop email client (MUA) and whilst it is sometimes possible to use it for other purposes, such as sending from a database, this non-mainstream use may attract little or no official support from the provider. They may even stop offering SMTP to end-users altogether in future. A dedicated mail-sending SMTP product would be a better bet for the long term.