Unix has a slightly odd way of handling file access permissions, inthat they're only checked when a file is opened, not every time thatyou attempt to do something with the file handle. While the choice isdefensible, in many ways checking permissions on access would be moreconvenient, since it would enable things like immediately revokingaccess to files, and very few programs actually care about this.

(And these days, it would fit better into network filesystems, manyof which operate this way anyways.)

Given modern email , one may sensibly ask what the roleof various sorts of autoresponders and other 'do things by email' thingsare. The simple answer is that they have no role; they cannot send emailto people who have not asked for it, plus pragmatically, most of theemail that they get will have forged sender information and be invalid.

Well, sort of, because the statement of the limitations also shows howyou can get around them. There are two general approaches.

First, you can simply