Remote access - Exchange server unavailable

1. Exchange server unavailable at times for remote users

I have setup my Exchange 5.5 server to allow client connections from
the internet. Mainly for the President of the company. He is using
Outlook XP. We had switched ISPs at the office, and he could not
connect anymore.

I added a entry to the lmhosts file and then it worked from the
internet. He then went on a business trip, and using a high speed
connection in the hotel he was staying in. This time, he could not
connect. Never even prompted for his username and password. He would
get the "exchange server is unavailable" message. He then fired it up
at home again and it worked.

He then came into work, connected to the LAN and he got the Exchange
server is unavailable messages again. I can't help thinking this is a
XP problem, but I have dial in users, you get the messages, click on
retry and then it connects.

Is this a Exchange server problem, or a Outlook client problem?

Can anybody shed some light on this? I've checked WINS, DNS, and the
firewall ports. All look in good shape. Sorry if this is not the right
group to be emailing with this question, but I'm stumped beyond
belief.

Thanks!

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