What could prevent mail user from receiving messages on mail client?

With SmarterMail v11.7.5157 on Windows 2008 Stadard

I have only one domain client who has a problem where his all employees can't login or receive incoming mails through his company network while all other companies don't have this issue.

Here is the scenario:

The client users connect and login and receive emails using Outlook fine through one of his none firewall networks and from any other home ADSL connection.
The client users cannot connect, login or receive messages from company domain using Outlook or any mail client using a network with Cisco hardware firewall but they still can send mails.
When we tried other two test domains (one one the same SmarterMail Server and another is none SmarterMail mail server) they only can login to second one and to first one from any other network but they can't from that network with the problem.
They receive fail message when trying login while test account settings and while send/receive (first 2 seconds) while all users are able to send mails.

Also the problem sometimes get solved when restart the mail server but back again in 5 minutes

I need to know if it's possible to be the Denial of Service (Dos) - POP 50 in 10 min settings effects the POP incoming service or might be a user network firewall issue?

I've changed to Denial of Service (Dos) - POP 150 in 10 min and they are working fine .. does this change effect my Abuse detection efficiency ? and what are the possible reasons that make their connections look as bad connections.

Is that mean anything to anyone?

Thanks in advance

Steve Reid Replied

10/2/2014 at 10:06 AM

If the whole office is using one IP then that could trigger abuse detection for sure.

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