Why can't POP3 clients receive Exchange Server email?

I can run an nslookup command and successfully connect to the outside email server (mail.badmc.org). However, when I try to connect to the outside email server from my Microsoft Outlook POP3 client, I receive a message saying the server was found but did not respond. Can this be done?

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If you have Microsoft Exchange deployed, you can have inbound mail delivered to Exchange mailboxes, rather than to your ISP. If users want to use POP3 clients to access their mailboxes, Exchange supports POP3. It's important to note that POP3 is a protocol to access email and mailboxes. POP3 clients use SMTP to send outbound mail to an SMTP server.

Not having a public external IP address complicates things a little. You can use a number of dynamic DNS services. These services install a light agent on the server, which connects to the DNS service provider and notifies it of its external IP address. The dynamic DNS provider updates the DNS zone for your domain with that IP address. This allows you to use MX records to receive inbound email, and also allows your POP3 clients, which are probably connecting from outside the firewall, to access your Exchange server using its fully qualified domain name (FQDN).

However, this arrangement becomes less practical in two scenarios:

1. If your ISP blocks the SMTP port, denying you the ability to send and receive mail using SMTP from a dynamic IP address to any other mail server on the Internet (except the ISP's own SMTP hosts). This is done for two reasons:
1. To deter spammers, and
2. To prevent customers using Internet connections generally meant for home use from using them as hosting servers. They want you to buy more expensive "business" connections. Nevertheless, you may find connections with at least one static IP address offered by your ISP at a nominal cost. If that's the case, I would recommend going with it.
2. Many mail systems, including those run by large service providers and Web-based email providers, block SMTP connections from dynamic IP addresses. Additionally, entire blocks of static IP addresses may be listed on some DNS Block Lists (RBLs).

Mixing an internal Exchange Server deployment with external ISP-hosted POP mailboxes adds more complications to your deployment. There are some POP3 connectors available for Exchange that allow your Exchange server to fetch users' mail from external POP3 mailboxes and deliver them to Exchange mailboxes, including the POP3 Connector available in Microsoft's Windows Small Business Server (SBS). I am not a big fan of these.

In your case, I would recommend getting Internet connectivity with a static external, public IP address if possible. If you do get one, simply point your MX record to your own external IP address.

If that's not possible, perhaps look at using your ISP as an SMTP relay.

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