Figure out implications of `{{ domain }}`

[taoeffect]

It would be ideal to not "brand" the machine with domain as defined in vars/user.yml, but instead simply have a list of domains with no "primary" one.

If a primary one is required, its implications (for example, the fact that using mail from the terminal will have a FROM: header of {{ domain }}) need to be thoroughly mapped out and documented. Any unnecessary externalities must be eliminated.

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