The "SMTP Receiving" tab allows you to configure parameters relative to this specific service's configuration, to add listeners, and manage access to the service by adding rules applicable to all existing listeners.
Listeners and control rules
Listeners
The first section in the SMTP Receiving tab is "Listeners". In this section you can add, delete, or edit the TCP socket listeners for the SMTP Receiving service.
Editing one of the existing listeners will result in accessing two configuration pages: "General" and "SSL Settings". The same pages will also be displayed when hitting the "Add Listener" button and choosing the "Advanced Config" option.
By default the listener will be configured on 127.0.0.1 (localhost) and port 25, which is the standard port (non-SSL) for SMTP communication.
Access control
Access rules allow you to control connection to this service from specific Networks / IP Ranges / IPs.
Flow control
Service configuration
The service flow control context allows you to define the max connections, max connections / interval, max peer connections, max peer connections / interval values.
Logging
You can select several types of messages to be logged for the SMTP Receiving service: critical messages, error messages, warning messages, informational messages and protocol communication. To select which of these are to be logged, click the "Log Level" slider and move it to the left or to the right. The selected types of messages will change color from whiter to blue.
Log files can be stored using your internal log files, your system's log files or within the log files located on a remote system. Use the "Log" drop-down menu to select where to have your log files saved.
Email loop protection
A looping message is an email sent from one mail server to another, without reaching its destination. Whenever it is received by a mail server, the email message will have a received header added. To prevent such email from increasing your mail server's traffic, check the "Email Loop Protection" option and set a number of maximum received headers for all received emails. This option allows you to avoid an infinite email loop that may be caused by a faulty configuration. Values range from 1 to 1000, however the default 30 value is recommended.
Error control
To set a maximum allowed number of errors caused by invalid commands received from clients or by failed authentication attempts, check the related options in the Error Control area. Use the up and down arrows corresponding to each of these options to set a specific number of errors.
A value of 0 means that the respective limit is disabled (no limit is imposed).
Thread management
Thread management allows you to set different numbers of processing threads for the SMTP Receiving service, depending on your traffic load. First, using the up and down arrows, set a number of threads to be allotted when the SMTP Receiving service is started. To have a different number of threads for peak periods, check the overload option and use the up and down arrows to choose the thread number.
Depending on your SMTP Receiving service load you can increase the number of processing threads which can lead to increased performance and response time for this service. However this increase should be made very carefully and making sure that the server hardware can provide the processing and memory resources required by a higher number of threads. The default setting of 2 starting threads and an increase on overload to 20 threads is recommended.
When you are done configuring these parameters, remember to hit the "Save Configuration" button to preserve your changes.