GT.M does not buffer mailbox output into logical records but instead sends all characters directly to the mailbox. When a process attempts to WRITE to a mailbox that does not have room to receive the message, VMS places the process in a resource wait state. A process in a resource wait state does not respond to <CTRL-C>. For such a process to resume processing, another process must READ one or more messages from the full mailbox.

Only GT.M format control characters manipulate the virtual cursor position. Mailboxes do not recognize the WRAP deviceparameter, nor are they subject to character filtering. WRITE increments $X for every character sent to a mailbox. READ* and fixed length READs increments $X for every character read from a mailbox. A normal READ sets $X to zero and increments $Y.

GT.M format control characters send ASCII representations of the function to the mailbox (for example W ! sends the ASCII characters <CR> <LF> to the mailbox).