On mail principles

[a at zeos.net]

A brief:

How to use "envelope from" provided that I have a mailbox on ISP.

A full explanation:

My computer has no static IP-address, nor DNS-name in Internet.
Internally, it has static name "localhost" and static IP-address 127.0.0.1.

So, I have a mail address "elisej at localhost".
But I cannot send a mail to the world using this address in "envelope from"
because of
1) Internet MTAs cancel mail whith such an address in "envelope from";
2) such an address is useless to recipient.

My address in the world is "a at zeos.net".
So, I make my MUA to send a letter using a command
setting "envelope from" to "a at zeos.net".
(like 
sendmail -f a at zeos.net
for sendmail, see "X-Authentication-Warning" in the header of this mail)
But this way is bad for local purposes: my MTA's diagnostic messages are sent
trough internet or lost at all, e. g. when my ADSL is in down.
(Moreover, my letters sent to other local users have non-local "envelope from"
address. Then local mail begin to go through external MTA.)

What to do in such a situation?

In principle, I can write a ruleset (in sendmail.cf by hand)
for my sendmail to rewrite
"envelope from" in outgouing mail from "elisej at localhost" to "a at zeos.net"
and to leave it unchanged in local mail.

But I think this is not a good solution, is it?

I use sendmail 8.13.6 on FreeBSD 6.1-RELEASE,
but the problem depends not on MTA nor on OS.

Elisej Babenko