I managed to succeed with OVH, you have to add some things.
With a pro mail server (fr - uk)
DISCOURSE_SMTP_ADDRESS: pro1.mail.ovh.net
DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: email@forum.com
DISCOURSE_SMTP_PASSWORD: "password"
DISCOURSE_SMTP_ENABLE_START_TLS: true # (optional, default true)
DISCOURSE_SMTP_AUTHENTICATION: login # (you need to add this line)
If you have a classic web hosting alongside the forum
DISCOURSE_SMTP_ADDRESS: ssl0.ovh.net
DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: email@forum.com
DISCOURSE_SMTP_PASSWORD: "password"
#DISCOURSE_SMTP_ENABLE_START_TLS: true # (optional, default true)
But if you have a dedicated smtp server, can’t help, sorry
It should work