On the module's main page, click on the Sendmail Options icon. A form for editing global options (shown in Figure 37.3) will be displayed.

When the Send outgoing mail via host field is set to Deliver directly, Sendmail will look up the correct mail server for non-local messages and connect to it to deliver email. If you select the second option and enter a hostname into the text box, however, all messages except those for local users will be send to that host instead. This will typically be a central mail server at your ISP or on your company's network.

If a domain routing rule exists for an address, it will take precedence over any server set using in this field.

The Delivery mode field controls how Sendmail processes incoming messages. The available options are:

Background or Default In this mode, email is immediately accepted from clients and then delivered to the destination by a separate background process. This is usually what you want.

Queue only or Deferred When one of these modes is chosen, Sendmail will simply add incoming messages to its mail queue. Only when the queue is explicitly flushed will they be sent to their destinations. This can be useful if your system is not always connected to the Internet, or if you want explicit control over when the server sends out messages. When a message is received in either of these modes, Sendmail performs no validation of the source or destination address, which would otherwise cause problems if your system is disconnected from the network.

Interactive This mode is similar to Background except that email is delivered by the same process that accepts it from the client. This means that clients must wait until Sendmail has transferred their messages to the next or destination server, which may take some time. It does, however, cut down the number of processes that Sendmail needs to start.

The SMTP port options field can be used to set the TCP ports that Sendmail listens on for SMTP connections. The actual value that can be entered can be quite complex, but if you want your system to accept connections from anywhere on the standard SMTP port, you should enter just Name=MTA. On some operating systems, this is not the default and Sendmail will only accept connections from localhost.

The Max load average for sending field can be used to set a load average above which Sendmail will not send out messages. The load average is a rough estimate of the number of processes running on your system at a moment in time, as explained in Chapter 11. This option and the next are useful for limiting the amount of CPU time that Sendmail can use up on your system, although the latter is more useful.

If Default is selected, the server will continue to process the mail queue and send messages no matter what the load average is.

To set a load average above which Sendmail will no longer accept new messages, enter a value in the Max load average for receiving field. When this level is reached, the server will close the SMTP port until the load drops back below it again. Typically, whatever you enter should be lower than the limit set for the Max load average for sending field, so that the mail queue is still processed even when the load is high.

When Default is selected, Sendmail will accept new connections regardless of the load average.

The Time before giving up field specifies the amount of time that Sendmail will attempt to send an email to an uncontactable remote server for before returning a bounce message to the sender. The value you enter must be a number followed by d (for days) or h (for hours). It should only be changed if your system is likely to be disconnected from the Internet for longer than the default of 5 days and you don't want messages in the queue to bounce.

The similar Time before sending warning field specifies the time that Sendmail will hold a message in the queue before sending a warning to the original sender. If your system is a secondary mail server for some domain or is often disconnected from the network, it should be increased to the same time as the previous field.

To change the address to which Sendmail sends fatal or double-bounced messages, enter a new address in the Send error messages to field. When Postmaster is selected, they will be sent to the Postmaster mailbox instead, which is usually an alias for root.

To limit the amount of disk space that Sendmail will use up for queued messages, fill in the Min free disk space field. If the number of free blocks drops below this level, new incoming messages will no longer be accepted. The exact size of a block depends on the type of filesystem in use, but they are typically 1 kB or 512 bytes in size.

To stop large messages being sent via your mail server, fill in the Max message size field. Any email larger than the number of bytes entered will be rejected when it is received. If you have a slow network connection and untrusted client users, this option can be useful for saving bandwidth.

Finally, click Save and Apply to save the new global options. Webmin will automatically restart Sendmail for you to activate them.