[now historical] An old-style UUCP electronic-mail address specifying hops to get from some assumed-reachable location to the addressee, so called because each hop is signified by a bang sign. Thus, for example, the path ...!bigsite!foovax!barbox!me directs people to route their mail to machine bigsite (presumably a well-known location accessible to everybody) and from there through the machine foovax to the account of user me on barbox.
In the bad old days of not so long ago, before autorouting mailers and Internet became commonplace, people often published compound bang addresses using the { } convention (see glob) to give paths from several big machines, in the hopes that one's correspondent might be able to get mail to one of them reliably (example: ...!{seismo, ut-sally, ihnp4!rice!beta!gamma!me}). Bang paths of 8 to 10 hops were not uncommon. Late-night dial-up UUCP links would cause week-long transmission times. Bang paths were often selected by both transmission time and reliability, as messages would not infrequently get lost. See the network and sitename.