[Maybe OT] SMTP mailer and authenticated use

[mike at pelleys.com]

Folks - I'm not sure if this is really on topic, but here's my problem.  I
currently run my own mail server from home and, of course, I have no
issues with providing anti-spam, anti-relaying, etc.  However, my ISP is
probably going to switch me to a port-restricted PPPoE DSL connection. 
Thus, I'm not going to be able to run mail mail server.

I do have the ability to put my server in a friend's server room but how
do I ensure that only "my" computers have the ability use the SMTP Postfix
server to send mail?  I mean, the server has to be able to receive
legitimate SMTP mail, but I don't want anyone else to be able to relay
from my server.  Since I will have a *very* dynamic (apparently) IP
address, I can't use the IP address restrictions within the main.cf to
address what IPs can use the server to relay.  I thought about SMTP SASL
authentication but it seems to me (please correct me if I'm wrong) that
this will block legitimate incoming mail to myself - won't it?

Anyway, if anyone has suggestions or point me to the right resouce, it
would be greatly appreciated!

Cheers,
Mike