> But I can't understand why it doesn't work if you put the \s* outside the parens, nor I can understand why it stops working if I put the \s*$ at the end of the regex :-(

That is because if you have the string

root: admin@somewhere.here
11111233333333333333333333
[download]

and the RE

/^root:\s*(?!\s*admin\@somewhere\.here)/
 ABBBBBCCC
[download]

then the part A in the RE matches the beginning of the string, part BBBBB matches 11111 ("root:") and CCC matches an empty string (not a space, a string with zero chars in it). After this empty string follows a space, and the space is not the beginning of "admin@somewhere.here", because it is the beginning of " admin@somewhere.here".

I hope things are getting clearer for you :-)