Mail was particularly prone to this because, in the days when mail was usually locally delivered to the UNIX system you'd also use for running interactive progr

Mail was particularly prone to this because, in the days when mail was usually locally delivered to the UNIX system you'd also use for running interactive programs, adding the ability for a program to read mail was just a fairly simple matter of parsing the user's mbox file. Most terminal mode IRC clients will at least tell you when you have new (locally-delivered) mail, for example.