Sounds like you need two line changes in your app.yml. Assuming you already have ssh access and are using the standard path and filename (/var/discourse/containers/app.yml):
DISCOURSE_SMTP_PORT: 587
to:
DISCOURSE_SMTP_PORT: 25
(which you’ve done by the sounds)
And uncomment:
#DISCOURSE_SMTP_ENABLE_START_TLS: true
and change to
DISCOURSE_SMTP_ENABLE_START_TLS: **false**