Perhaps there is a type of firewall device between your systems and the Mdaemon server that's blocking TCP 25 (SMTP) connections. If a firewall is configured correctly, only the internal mail server should be able to send Email to the outside world while all other internal systems will not be able to.
Additionally, most workstation anti-virus clients will intercept outbound TCP 25 connections "assuming" that the workstation is infected and attempting to send spam if it's trying to send on TCP 25.
Hope this helps...
Mike