I’m assuming by “dot” you meant @
In fact both are optional. With FQDN-Adressing a user without domain defaults to localhost, with Bang-Adressing there is no @ because the last system is left for interpretation of the last receiver and if he consideres it a user, so be it.
I beat that. When I first installed a BSD in 1988 - I was 16 and stupid - I thought I would copy files like on Amiga or MSDOS by “copy filename c:” and did cp filename /dev/sda.
You only do that ONCE.