SMTP server settings protect the server on which Kerio MailServer is running from misuse.
Protection of the SMTP server enables users to define who will be allowed to use this server and what actions he/she can perform. This way, the server is protected from being misused. If the SMTP server is available from the Internet (anytime when at least one MX record is directed to it and the port 25 is available for access), any client can connect and use the server to send an email message. Thus the server can be misused to send spam messages. Recipients of such email messages will see your SMTP server as the sender in the source text and might block receiving messages sent from this server. Thus your company might be considered a spam sender and your server can be added to a database of spam servers.
Kerio MailServer provides a protection system that enables users to define who will be allowed to send email via this server and where. Anyone can connect to the SMTP server to send messages to local domains. However, only authorized users will be allowed to send email to other domains.
In this section, the delivery parameters can be also set:
Use the Relay control tab to set groups of allowed IP addresses and/or user authentication against SMTP server.
Use this option to activate user authentication by IP addresses or usernames and passwords (see below). Generally, authenticated senders can use email messages to any domain via this server, whereas unauthorized users can send messages only to local domains.
Also add all trustworthy servers to this IP group. These servers will not be checked by the SPF and Caller ID modules (for details, see chapter 13.5 Email policy records check). Trusted servers will not be even checked by SpamAssassin. However, this filter can be enabled by a special option in the Spam Filter section on the Spam Rating tab if necessary (for more information, refer to chapter 13.1 Spam Rating tab).
Use this option to define a group of IP addresses from which email can be sent to any domain. Use the IP address group menu to choose an item from the list of groups defined in . Use the button to edit a selected group or to create a new one (see chapter 19.1 IP Address Groups).
Users authenticated through SMTP server using a valid username and password will be allowed to send email to any domain. Thus, all users that have their own accounts in Kerio MailServer will have this right.
Users authenticated through POP3 (username and password) will be granted relay access from their IP address for a period of time given in the Allow SMTP relay for ... minutes after successful POP3 login field.
Authentication by IP addresses is independent from authentication by usernames; therefore users must meet at least one of these conditions. If both Users from IP address group and Users authenticated through SMTP server... options are selected and the SMTP authentication fails, Kerio MailServer does not verify, if the user belongs to the allowed IP addresses.
In this mode, the SMTP server does not check users who use it to send email. Thus any user can send email messages to any domain.
We recommend you not to use this mode if Kerio MailServer is available from the Internet (i.e. it uses a public IP address and port 25
is not blocked by a firewall). If Kerio MailServer is available from the Internet and uses a public IP address with port 25
not behind the firewall, it is highly probable that it will be misused to send spam. This could overload your Internet connection. This might also cause that your server will be included in databases of spammer SMTP servers (see below).
Apart from completely blocking certain senders Kerio MailServer provides options that limit, for example, sending too many messages or opening too many connections (known as DoS attack. These options can be set in the Security Options section.
Maximum count of messages that can be sent from one IP address per hour. This protects the disk memory from overload by too many messages (often identical and undesirable).
Note: Maximum count of messages received from a single IP address is checked always for the last hour. If this option is enabled, any new message sent from the IP address where the limit was exceeded in the recent our is discarded.
Maximum number of concurrent TCP connections to the SMTP server from one IP address. This is a method of protection against DoS attacks (Denial of Service — too many concurrent connections overload the system and no other users can connect to the server).
Also known as a Directory harvest attack, this condition is met when an application that guesses common usernames of recipients' fails up to the number of allowed unknown recipients. If this type of protection is enabled, the server sending messages to an unknown recipient is blocked for an hour.
Group of IP addresses on which the limitations will not be applied. This rule is often used for groups of local users (see the Relay Control tab). These users send all their outgoing mail through Kerio MailServer — the count of messages sent by these users to this server is therefore much higher than the number of messages sent by external users (servers) that use it only to deliver mail to local domains.
It is also recommended to include the secondary SMTP server to the list of allowed IP addresses, because in some cases, its behavior can be similar to that of an attacking server.
When a message is received Kerio MailServer checks whether the sender's domain has a record in DNS. If not, the message will be rejected. This feature protects from senders with fictional email addresses.
Note: This function may slow down Kerio MailServer (responses of DNS servers may take up to several seconds).
Maximum number of message recipients that will be accepted (in number of Rcpt
commands in the SMTP envelope).
Spam is often sent by special applications that connect to SMTP servers and ignore its error reports. If this option is enabled, Kerio MailServer will close the SMTP connection automatically after the defined number of failed commands has been expired.
Maximum size of a message that will be accepted by the SMTP server. This protects the server from being overloaded by large messages, therefore we strongly recommend to activate this option. The 0 value means that no limitation is set. For easy definition you can switch between kilobytes (KB) and megabytes (MB).
This parameter helps the server block messages that have been trapped in a loop.
In this section, the delivery parameters can be also set:
Mail will be delivered directly to destination domains using MX records.
All outgoing mail will be sent via another relay SMTP server.
DNS name or IP address of relay SMTP server.
Port where the relay SMTP is running. Typically the standard port 25
is used (this value is also set as ).
Use this option if relay server requires authentication of sender (Kerio MailServer) using username and password. Specify the User and Password entries.
A method used for authentication at the parent server: SMTP AUTH Command or POP3 before SMTP.
First, the user authenticates to the POP3 account at the server. After this authentication the user is known already and they can send email via the SMTP server. Username and password used here will be used to login to the mailbox and no messages can be read. Therefore you do not need to define mailbox in
to send an email message.)When sending a message, SMTP server attempts to use encrypted connection first (SSL). If SSL connection is not supported, unencrypted connection will be used. Thus the maximal possible security of sent messages is ensured.
In this tab, mail queue can be set. It can be viewed in
.Maximum number of delivery threads that will send messages from the queue (maximum count of messages sent at one moment). The value should be chosen with respect to processor capacity and to speed of the Internet connection.
Interval that will be used for repeated retry attempts for sending an email message.
If the message is not delivered in the time defined, it will be discarded and its header including DSN will bounce to the sender. It will be also automatically removed from the queue and no more delivery attempts will be taken by the server.
You can also use preset time units (minutes, hours, days) to specify the interval.
However, these time units will not be considered if the messages are delivered via relay SMTP server.
If the message could not be delivered by expiration of this period, sender will be sent a warning (server will continue in sending attempts).
Language that will be used for error, warning and informative reports.
Note: Reports are stored in the reports
subdirectory of the directory where Kerio MailServer is installed (UTF-8 coding is used). Administrator can modify individual reports or add a new language report version.