I am experiencing mailbox locking problems with Sun's mailtool used over a network.

Under the Expert settings of mailtool is a option to turn on Use network aware mail file locking. By default dtmail has this set, but mailtool doesn't. You should set it. The help info on dtmail has this to say about it: Mailer tries to prevent two different instances of itself from opening the same mail file at the same time through a technique that detects this access when both instances of Mailer and the file are all on the same machine. A network-aware mail file locking protocol is available that uses ToolTalk_ to coordinate instances of Mailer running from more than one machine, or mail files accessed over the network. Mailer can only change this option when first opening a mail file. If you are using the SunOS4 version of mailtool, this apparently doesn't work. The only thing which does seem to work it getting the user to hit the done button to make it release the lock.