Send Error: What exactly is relaying

[Xaos]

Lately, I have been getting this message when trying to email from home,
using my work email address:

Your message did not reach some or all of the intended recipients.

Subject: RE: CARFAX Hot Lead
Sent: 1/31/2005 8:48 AM

The following recipient(s) could not be reached:

'(e-mail address removed)' on 1/31/2005 8:48 AM
530 5.7.1 Relaying not allowed: (And then is lists the email
address here)

It happens regardless of who I email. What exactly is relaying?

Thanks in advance!

Chaz