[lug] multiple relayhosts (postfix)

[Ben]

I'm running postfix on our intranet and it sends all outgoing mail to an 
smtp server with the config line:
relayhost = smtp.isp.com

Our relayhost doesn't like large e-mails -- it just drops the connection 
after 95% transmission and then postfix tries again since it never got 
an error, just a dropped connection. The second attempt fails the same 
way, and I've got an endless loop. A friend of mine opened up his smtp 
server to me and I'd like to use it for large e-mails, but not for just 
regular e-mails.

So, can I either:

a) have postfix use two relayhost, one primary and one if the first one 
fails

b) configure postfix to send all e-mails over 10M to a second relayhost?

Maybe I missed it in the documentation, but I can't find anything about 
setting multiple relayhosts (the relayhost line is a default route and 
only take one address). And I can see how to specific what relayhost to 
use based on destination of e-mail, etc, but nothing based on e-mail size.

Thanks,

Ben