Bitwarden’s SMTP service does behave a bit weird without some settings in the /opt/bitwarden/bwdata/env/global.override.env file.
For allow TLS on port 25 use the option „globalSettings__mail__smtp__sslOverride=true“.
For not using SSL and fix the mail service to TLS use the option „globalSettings__mail__smtp__ssl=false“.
You can find more information in the logfiles in /opt/bitwarden/bwdata/logs/events.
Example:
globalSettings__mail__replyToEmail=sender@domain.tld
globalSettings__mail__smtp__host=relay.domain.tld
globalSettings__mail__smtp__port=25
globalSettings__mail__smtp__ssl=false
globalSettings__mail__smtp__username=<not needed for open relay>
globalSettings__mail__smtp__password=<not needed for open relay>
globalSettings__disableUserRegistration=true
adminSettings__admins=bitwarden-admin@domain.tld
globalSettings__mail__smtp__sslOverride=true