A genuine multitasking system like Linux runs many processes concurrently. Programs must compete for data access. Assigning locks to files ensures exclusive access and prevents the possibility of data.
Most Linux machines have an MTA, a Mail Transfer Agent. This can be Postfix, Exim or even Sendmail. The MTA either uses a Fetchmail process or TCP to fetch email messages. When the MTA finds a message addressed to the local user, it passes the message on to the Local Delivery Agent (LDA). And the LDA stores the message in a mailbox file, after possibly taking a detour via a filter such as Procmail. If two messages arrive at the same time, the MTA will hand both of them to an LDA process at the same (see Figure 1). Each process will then attempt to write to the same mailbox file, again at the same time. If you are lucky, the messages end up in the right file, but in the wrong order, but you are far more likely to lose a file as the processes overwrite each other’s data.
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