Ordinarily (beginning with V8.6 sendmail), there is no limit to the number of queued messages that can be processed during a single queue run. If there are more messages than sendmail has allocated memory for, sendmail will calmly allocate more memory. (Previously, a fixed limit was imposed at compile time.)

Some systems process so much mail that a single queue run can become unmanageably large—so huge, in fact, that system resources are strained to the limit with an adverse effect on system performance. If your site suffers from this problem, beginning with V8.7 you can set an upper limit on the number of queued messages to be processed by using the MaxQueueRunSize option:

O MaxQueueRunSize=limit                  configuration file (V8.7 and later) 
-OMaxQueueRunSize=limit                  command line (V8.7 and later) 
define(`confMAX_QUEUE_RUN_SIZE',limit)   mc configuration (V8.7 and later) 

Here, limit is of type numeric and defines the upper limit on how many queued messages can be processed during a single queue run. If limit is less than or equal to zero, if it is missing, or if the entire option is missing, no limit is imposed. The default is to impose no limit.

If MaxQueueRunSize is defined and if that limit is reached while processing the queue, sendmail will log the following message at LOG_ALERT:

WorkList for queuedir maxed out at  limit  

Processing of the queue is described in Section 11.7.

The MaxQueueRunSize option is safe. Even if it is specified from the command line, sendmail retains its special privileges.