SMTP sessions hang at "transfer complete"
Outbound sessions hang at 'Transfer complete.'
If your MDaemon is not able to send outbound mail with the above-described symptom, check your firewall setting, if you have one.
Make sure that your firewall is set to allow ICMP traffic. In some cases ACK traffic will not come through the firewall and reach MDaemon with ICMP traffic denied. MDaemon must receive the ACKs to know that the message has been received.
It may also help to adjust the router MTU from what is typically default of 1500 to something like 1200. The optimal setting may be as low as 800, but 1200 will work in most cases.
The MTU setting is in your router - MDaemon cannot control this configuration.