Crash mail
FidoNet mail sent straight to the recipient, right now, without waiting for the nightly route — and on the sender's phone bill.
Ordinary netmail was routed through the network's hosts and hubs and went out during Zone Mail Hour, when calls were cheap and every node had promised to answer. Crash was a flag on the message that bypassed all of that: the mailer looked the recipient up in the nodelist and dialled directly, whatever the time of day. The other flavours were Hold, where mail sat waiting until the recipient called in to collect it, and Direct, which also went straight there but without the urgency.
In mailers like BinkleyTerm the flag showed up in the outbound filename: .CUT for crash, .HUT for hold, .OUT for normal. Since a crash call to the other side of the planet cost real money, it wasn't something you did to say hello. It was for sysop business that couldn't wait a week.