Outgoing Mail problem

We have been travelling for several weeks.

We took along our Macbook and it worked at every location where we stayed.

Now we are home and we have a challenge. the outgoing Mail will not send from the MacBook

We can receive incoming mail and Safari is working.

We get the usual message re the server and "try later"

Both our iMac and the MacBook pro are working.

MobileMe is working and we can send and receive Mail via MM on Safari on the MacBook

I have checked the settings,dignostics and have compared the settings to both the IMac and the MBP. All appear to be correct.

Closed Mail, rebooted hot and cold, left the e Mail to be sent running overnight and still not sent, just resident in the Outbox.

I suspect a Port problem but I am a non tech person and that's as far as I can go.

Any suggestions and help, as usual is appreciated.