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How do postfix and mailx interact with mbox locking?

If postfix is setup as the MTA using its virtual transport to deliver mail to a local mbox file (and handling delivery of outgoing messages), and mailx is the MUA, being used on that same computer for reading and composing messages, can only one of these operations happen at once due to mbox file locking?

If a user is using mailx to read their messages and perhaps reply to or compose a few messages, does this prevent postfix from delivering new messages to the user's mbox file? Does this create a situation where the user might not receive some messages due to the incoming messages being bounced?

What is the process for mbox file locking here? When and where is it happening? I am trying to get a better idea of how this works.