If you are getting warning messages during mail send that indicate ListManager is running low on file descriptors, you may need to limit the number of simultaneous sockets.
Windows
1. Open your lmcfg.txt file.
2. The default setting for the $max_socket_connections variable is 3000. Raise it to 3500. If this does not help, try raising it again until mail send improves.
Linux
1. Open the S96lm script.
2. Raise the ulimit -n amount by 500. If this does not help, try raising it again until mail send improves.
NOTE If you are running low on memory and experiencing stability problems, you may need to lower the above amounts.