Apologies, I do have this working now. My test was insufficient. I was expecting an email when a task is assigned but that seems not to be a feature. I am now receiving emails for reminders. Microsoft 365 mailing does work with the following settings.
VIKUNJA_SERVICE_ENABLEEMAILREMINDERS: 1
VIKUNJA_MAILER_ENABLED: 1
VIKUNJA_MAILER_FORCESSL: 0
VIKUNJA_MAILER_HOST: smtp-mail.outlook.com
VIKUNJA_MAILER_PORT: 587
VIKUNJA_MAILER_USERNAME: xx@xx.xx
VIKUNJA_MAILER_PASSWORD: xx
VIKUNJA_MAILER_AUTHTYPE: login
VIKUNJA_MAILER_FROMEMAIL: xx@xx.xx