[exim] "Can't assign requested address" on outgoing email.

Hi all,

All of a sudden, for no reason I can find, I am unable to send to some
domains. I am getting errors like this. I am trying to figure out how to
troubleshoot the error message:

"Can't Assign requested address"

2009-08-07 15:49:54 1MZVRp-000Mpn-Fp <= sims@this server.com
H=d221-76-81.commercial.cgocable.net (minePC) [216.221.76.81]
I=[ip.of.this.server]:25 P=esmtpa A=login:sims@??? S=42381
id=2BD0634A21604819A82AF78279D8ABE1@minePC T="Re: Something" from
<sims@???> for THood@???
2009-08-07 15:49:55 1MZVRp-000Mpn-Fp sbpartners.ca.s7a1.psmtp.com
[64.18.6.14] Can't assign requested address

I have my exim setup so that authenticated user's (A=login) is sent from
thier own IP (each domain on the server have a public IP). This has worked
very well until now.

In this case, is the 'Can't assign.." refering to the ip of the sending
server or the receiving server?

-Grant