LinuxQuestions.org - Postfix on dial-up, single-user machine

Hi,
I finally managed to set up postfix, fetchmail, procmail an mutt to work with each other, on stand-alone dial-up machine (no networks, gateways or something else, just stand-alone single user machine). Well actually, I've delayed mutt configuration because of the one additional problem I have with postfix, that I would like to ask you for advice.

Postfix is, of course, installed and working, but only with one account. That is, (by using "Installing a sophisticated e-mail environment for systems with only a dial-up Internet connection" tutorial) I added to main.cf the following:
#my hostname, yes it's slackware distro
myhostname = darkstar
myorigin = $myhostname
mydestination = darkstar.example.net, localhost.example.net, localhost.localdomain, darkstar, darkstar.localdomain, localhost
mynetworks = 127.0.0.0/8, 192.168.1.0/24
#my isp's server for sending mail through
relayhost = mail.gmx.net
sender_canonical_maps = hash:/etc/postfix/sender_canonical
alias_maps = hash:/etc/postfix/aliases
mail_spool_directory = /var/spool/mail

and made sender_canonical in following way:
axa_aaa@darkstar.example.net user_axa@gmx.net
axa_aaa@localhost.example.net user_axa@gmx.net
axa_aaa@darkstar.localdomain user_axa@gmx.net
axa_aaa@localhost.localdomain user_axa@gmx.net

Of course, after that, I run postmap for both sender_canonical and aliases (it was not in ndb format)

And, as already written, it is working as expected, but, again only with single account.
I have two more accounts at different servers that I would like to use. Question is, how can I set up postfix, so I can send my mail through one of the following servers:
mail.gmx.net
smtp.hotpop.com
smtp.some_other_host.com

and change my envelope address from axa_aaa@darkstar.example.net to respectively to:
user_axa@gmx.net
axa_user@hotpop.com
user_axa_aaa@some_other_host.com

??

Should I try to add all these servers in main.cf like:
relayhost = mail.gmx.net
relayhost = smtp.hotpop.com
relayhost = smtp.some_other_host.com

Then delete file sender_canonical, and then finally, when I want to send my mail, through mutt (or some other mua) to specifically assign _from_ field? Would this work in desired way, or perhaps, there is some more "elegant" way for handling this kind of situation?

Thank you.