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Setting up a Mail Server even if your ISP is making it hard to do so

Sometimes ISPs make it difficult to run an email server. This guide shows you how to leverage ClearOS and ClearCenter to get around those issues. We often see people who want to run mail servers come with difficulties from their ISPs that make it less than hospitable to do so.

Technology to the Rescue

When properly used, the following services overcome a whole host of issues. Below is the ClearCenter technology and the issues that are resolved:

DNS Hosting

When ClearCenter is the registrar and hosts the DNS for a client, we are able to do two things:

  • First, we provide MX Backup for the client which means that if the ISP goes down, changes the IP address on the client or if the server is temporarily down, the mail will pool on ClearCenter servers until such time that it can deliver.
  • Second, by hosting DNS with ClearCenter, Dynamic DNS is opened to the client so that their own DNS names can appear end to end for resolution to MX hosts.

To get started using DNS Hosting from ClearCenter:

Dynamic DNS

By far, this is the most powerful service for compensating for uncooperative ISPs. Dynamic DNS is able to assign a name to a host whose IP address changes from time to time. This avoids a typical fee for static IP addresses. The DNS resolution provided for free by ClearCenter has a 5 minute time to live. Which means that your mail would only be delayed by a few minutes if the server reboots and finds itself on a new IP or if the ISP swaps the IP address on the client mid-stream.

Dynamic DNS is activated by installing and configuring the Dynamic DNS App from the ClearCenter Marketplace on your ClearOS server. Once installed, you can affect the way you want it to behave in your ClearSDN portal.


SPF records can tell the email world that your host is a reliable sender of mail. This can compensate for the fact that your host may not have reverse PTR records or happens to have a block of IP addresses that are well advertised to by dynamic IP addresses (such as AOL, or others). This is not a guarantee, but it is a good measure. Email servers may give particular weight to reverse PTR records but SPF is recognized more and more as an effective way to cut down on unwanted email (so that your mail server isn't mistaken for a spammer).

While ClearOS doesn't make this happen at the server, it does happen at the DNS servers. You can create the SPF record in your DNS manager at ClearCenter. Or your other DNS provider.

Mail Forwarding

Some ISPs are so toxic to running a mail server that they will block port 25 email either outbound or inbound thus blocking the ability of a client's site to become a mail server. ClearOS has ways around this but will need more help from the outside:

Inbound mail

If the ISP is blocking the ability for your ClearOS server to receive port 25 traffic, you can get around this by having ClearOS run on a port other than port 25. Additionally, you will need a host elsewhere on the Internet capable of receiving mail on the standard port (25) and configured to forward the mail to that alternate port on your ClearOS server.

If the host elsewhere is another ClearOS server (eg. hosted in the cloud), simply set the domain for the client in the SMTP Server module's Destination Domain section (make sure that the domain for the client is NOT set in the Domain section above). Then add the domain, and the DNS or Dynamic DNS hostname of the server, and the new port to the Mail Forwarding section. This method provides a good opportunity for some Managed Service providers to ensure delivery, monitor uptime, and provide extra service to their clients.

In the case where no alternative server is available as a mail relay host, you will need to acquire an alternate SMTP port service from a provider. Examples:

Outbound mail

content/en_us/kb_o_setting_up_a_mail_server_even_if_your_isp_is_making_it_hard_to_do_so.txt · Last modified: 2017/07/24 06:56 by bchambers