Sending mail problems OSX 10.5.6

[Greater Pewee]

I have been having trouble sending emails out with Mail in OSX 10.5.4, 10.5.5 and 10.5.6. I have talked with Apple service and my email service provider without complete success in solving the problem. The problem is that emails come in alright but outgoing mails are intermittent. Sometime they wait all day to go out. After hitting the send button various messages saying that it could not connect to the SMPT server would appear. No amount of changing in the email preferences would help. I eventually discovered something by checking my network preferences. Both Airport and Ethernet were being used to connect to the internet. When I switched off Ethernet mails in the outbox went out right a way but subsequent mails hanged in the outbox. When I switched back to Airport and off the Ethernet the outbox mails went again right away. Now I am using Airport only but I have to turn it off after seeing the connection warning, wait a few seconds and turn Airport back on and the outbox mails will go out right away. I feel that there is something corrupt in my Network Preferences but am not sure how to fix it.