emacsclient, -a emacs, and --eval (rephrased)

[Florian v. Savigny]

Encouraged by Ronaldos post, I thought I might rephrase one aspect of
my problem, hoping this might lead to the (or some) solution:

When I call

emacsclient -a emacs --eval "(rmail)"

Emacs will start up and run rmail if it is started as emacsclient
(i.e. if emacs and its server are already running). But when started
as emacs, it starts by opening a new file called "(rmail)".

This is confusing to me, as the --eval option is documented for both
emacsclient and emacs, leading me to think both should interpret it in
the same way.

This seems basic enough - can anybody explain to me what I am missing
here? 

Thanks so much in advance!

Florian