Mail Tool tries to prevent two different instances of Mail Tool from opening the same mail file at the same time. By default, a technique that successfully detects this access when both Mail Tools and the file are all on the same machine.
As an option you can use a network aware locking protocol that uses ToolTalkTM to coordinate the Mail Tools. This ensures consistency if running MailTool from more than one machine, or if mail files are accessed over the network.
Mail Tool can only change this option when first opening a mail file. If you change this option when a mail file is open you are given three choices:
You can save the current mail file and reopen it, making the new locking technique take effect immediately.
You can continue editing the current mail file. The new locking technique will take effect the next time you open a mail file.
You can abort the apply and return to editing the property sheet.