SNDMSG

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NGC 6087 (Wikipedia) NGC 6087 (also known as Caldwell 89 or the S Normae Cluster) is an open cluster of 40 or more stars centered on the Cepheid variable S Normae in the constellation Norma. At a distance of about 3500 ly and covering a field of almost one quarter of a degree, the stars range from seventh- to eleventh-magnitude, the brightest being 6.5 magnitude S Normae. The aggregate visual magnitude of the cluster is about 5.4.

source code

https://github.com/PDP-10/tenex/blob/master/cusps/sndmsg.mac

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I was making improvements to the local inter-user mail program called SNDMSG. Single-computer electronic mail had existed since at least the early 1960’s and SNDMSG was an example of that. SNDMSG allowed a user to compose, address, and send a message to other users’ mailboxes.
A mailbox was simply a file with a particular name. It’s only special property was its protection which only allowed other users to append to the file. That is, they could write more material onto the end of the mailbox, but they couldn’t read or overwrite what was already there. The idea occurred to me that CPYNET could append material to a mailbox file just as readily as SNDMSG could. SNDMSG could easily incorporate the code from CPYNET and direct messages through a network connection to remote mailboxes in addition to appending messages to local mailbox files.
Next, the CPYNET code was incorporated into SNDMSG. It remained to provide a way to distinguish local mail from network mail. I chose to append an at sign and the host name to the user’s (login) name. I am frequently asked why I chose the at sign, but the at sign just makes sense. The purpose of the at sign (in English) was to indicate a unit price (for example, 10 items @ $1.95). I used the at sign to indicate that the user was “at” some other host rather than being local.
These first messages were sent in late 1971. The next release of TENEX went out in early 1972 and included the version of SNDMSG with network mail capabilities. The CPYNET protocol was soon replaced with a real file transfer protocol having specific mail handling features. Later, a number of more general mail protocols were developed.
The First Network Email (openmap.bbn.com)

Wikipedia

SNDMSG (Wikipedia) SNDMSG was an early electronic mail program, chiefly notable because it was used to send what is considered the first networked email.