sendmail, 4th Edition

ConnectionCacheSize

SMTP connection cache size V8.1 and later

Without a connection cache, sendmail uses a single autonomous SMTP session to transmit one email message to another host. It connects to the other host, transmits the message, and closes the connection. Although this approach is sufficient for most mail, there are times when sending multiple messages during a single connection is preferable. This is called caching connections.

When sendmail caches a connection, it connects to the host and transmits the mail message as usual. But instead of closing the connection, it keeps the connection open so that it can transmit additional mail messages without the additional overhead of opening and closing the connection each time. The ConnectionCacheSize option of V8 sendmail specifies that open connections to other hosts should be maintained, and it specifies the maximum number of those connections. The forms of the ConnectionCacheSize option are as follows:

O ConnectionCacheSize=num           ← configuration file (V8.7 and later)
-OConnectionCacheSize=num           ← command line (V8.7 and later)
define(`confMCI_CACHE_SIZE',num)    ← mc configuration (V8.7 and later)
Oknum                               ← configuration file (V8.6 and later)
-oknum                              ← command line (V8.6 and later)

Optional whitespace can precede the num. The num is an integer that specifies the maximum number of simultaneous connections to keep open. If num is zero, this caching feature is turned off. A value of 1 is good for workstations that forward all mail to a central ...