Occasionally the MTA is unable to deliver messages due to network connectivity issues. In these cases, the messages will be stored on the queue until the next time the MTA is able to attempt to deliver (as defined by the retry interval).
Planning on disk space for these outages is based on a simple rule, the “General Rule for Message Queue Sizing:”
Determine average number of messages/minute expected to be delivered (N).
Determine average size (kb) of messages (S).
Determine maximum duration (minutes) of typical network connectivity outages (T).
Thus, the formula for estimating the Disk Queue Size is:
Disk Queue Size (kb) = N x S x T