Mail won't connect to server.
Suddenly Mail will not work. When I try to send a message I am warned that the message cannot be sent with the selected server. When I use the connection doctor All accounts show green and have a message along side saying the connection and log in succeeded. I have verified the accounts settings and all are as recommended by Apple. Mail is working perfectly on my ipad, ipod Touch and my macbook. All have the same mail settings. I can use the web mail provided by my ISP as well. If I send a message to myself from any of my other devices it can be received on my iMac but I cannot forward or reply to this message on the iMac.
This is driving me mad! Has anyone got any ideas? I would be most grateful for any help.
iMac, Mac OS X (10.7.2), iMac 20