Outlook and Outlook Express intermittant problem

I am facing a strange problem.  We have hosted our mail server with an ISP.  We are connected  to internet through DSL connection provided by another ISP. For the past 2 days some of the mail accounts are giving problem. Either the SMTP or POP3 server getting timed out  for some of the accounts (when they use mail clients like outlook or outlook express) . The same users can send and receive mails through web access. If those accounts are configured in different systems also, they are giving error.

Other users do not have any problem even with the mail clients. All the users are connected to internet through same DSL connection and are connected to same LAN. If we check the problem giving accounts through a dialup connection , they work perfectly even in outlook. The same accounts  work once or twice through the same DSL connection immediately after we change the configuration. That is, when outlook is giving problem for an hour, I change the pop3 and smtp server names to a proxy server, it works. After some time, I change them back to original pop3 and smtp servers, it works once or twie. Again it gives error.

It can not be the system problem because in the same outlook, I have configured 2, 3 accounts out of which only one gives problem. DSL providers says that he can not be blamed because most of the accounts work and browsing is also ok.

Please let me know your suggestions.

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The problem is with the ISP hosting the mail accounts, probably.  Since the same accounts experience this problem on different machines, it's not a problem with your local computers.  Since other users can get their mail, and everyone can browse internet, it's not a problem with the DSL.  For some reason the server on which the problematic accounts are sitting is not responding to requests being made to ports 25, 110, or both.

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RachelbAuthor Commented:
But,   when we check those accounts through  dialup connection, they work.
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