Hi Steven,
I have the same problem.
SME 5.6U4, ADSL Static IP
Mail use to just scream out of the outbox, now it sits for anywhere from 30 seconds to 15 mins. (sometimes days) These emails are just text.
I had to increase the time out in outlook settings to send because it would just error out complaining that it could find the server, but the server didn't respond???
I tried the local ip and the FQDN for the mail server with the same results.
It seems that qmail (or smtpfront) isn't responding to the internal network SMTP request, or it is just taking its time...
Thanks
Shane