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SMTP Server

SMTP lets you can manage your own mail server. There are a number of reasons this might be advantageous:

  • Ability to have a customized user and domain name - for example,
  • Mailboxes limited only by hard disk storage capacity and your own administration settings
  • Alias support - for example, can be sent to and
  • No waiting around for new users to be added
  • Custom antispam control
  • Antivirus support
  • Privacy
  • Full control


A number of services are available for mail services:


If you did not select this module to be included during the installation process, you must first install the module.

You can find this feature in the menu system at the following location:

Server|Mail|SMTP Server


SMTP Mail Configuration

General Settings

The Hostname does not have to be related to the e-mail domains that you host. It can be ANY valid Internet name for your machine. For example, you may wish to have a dedicated mail server on your network. In this case, you might want to name this machine This would be the Hostname you would enter.

The Primary domain field indicates the domain name this server will act as an SMTP/Mail server for. If you have a single domain name that you receive mail for, enter the domain here.

If SMTP Authentication field is set to on, any client attempting to send mail through the server will require a username/password before accepting mail for delivery.

SMTP Authentication - Thunderbird

For Mozilla's Thunderbird, click on Tools|Account Settings and then click on the Outgoing Server (SMTP) field.

Ensure the Use name and password setting is checked and enter the username of the mail account in the username field. The password will be requested by the mail client application on the first attempt to send mail. There will be an option to save it to the Password Manager so that you do not have to enter each time you send mail through the server.

SMTP Authentication - MS Outlook/Outlook Express

If you are using MS Outlook/Outlook Express, click on Tools|Accounts. Select the account which will use this mail server to send mail and click on the .

Make sure the My server requires authentication is checked. Click on the button to enter the details of your username/password.

Setting the Catch All User to an valid user on the server will pass all mail sent to an “Unknown user” to this account. To bounce mail addressed to an invalid recipient, set to Return to sender.

Trusted Networks

A trusted network is a list of networks that are allowed to send mail through the SMTP server. Dynamic IP's should not be added to this list. It is important that you do not make an error with this parameter. The default setting allows any user with a 192.168.x.x address send e-mail through the server. If you use a 10.x.x.x address, you should add to the list of trusted networks.

Outbound Relay Hosts

Some ISPs will block all traffic on port 25 unless it it destined for their mail servers. In this case, you would want to specify your ISPs mail server as the Outbound Relay Hosts.

Additional Domains

Destination Domains

If your company/organization has multiple domains and you wish to receive email sent to any user for any of the domains, enter additional domains to the Destination Domains list. For example, if our primary domain was setup to be “” and we wanted all emails sent to the following registered domains to be valid:


We would add the domain list above to the “Destination” domains list.

Virtual Domains

Virtual domains are not supported; here's why.

Mail Forward Domain List

If you are configuring ClearOS to be an anti-spam and anti-virus engine for another production mail server, add the domain name to the “Mail Forward Domain list”. If the Mail Antispam module is installed and running on the server, mail will be subject to the spam identification rules you have configured. Similarly, if the Mail Antimalware module is installed and running, all mail for the domains will be scanned before passing the mail on to the destination production mail server specified in this field.

ClearOS will run filtration on both directions of the mail flow. This means you can set your internal mail server to use ClearOS as a outbound relay or smarthost that will filter for spam and viruses for outbound messages also. This can help prevent your mail users from spreading malicious mail if their workstations are compromised.



Do not forget to open up firewall ports for your e-mail server: port 25 on the firewall configuration page.

ISP Blocking

Some ISPs are known to block SMTP (port 25) traffic to residential broadband connections in an attempt to cut down on SPAM originating from their network. If you think your configuration is set-up correctly and you suspect your ISP is blocking SMTP traffic, try a port scan.

content/en_us/5_smtp_server.txt · Last modified: 2015/03/02 08:37 (external edit)