Hello,
I had the same problem - incomming (smtpd) TLS is working, OK. But when I send emails to TLS enabled recipients, there was no TLS used.
This settings fixed this issue for me through:
smtp_tls_security_level = may
smtp_tls_loglevel = 0
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
Is there any reason why is this disbled by default in iRedMail?