Hi Jon,
There are numerous ways like we discussed in this thread one is using an SPF record. SPF stands for Sender Policy Framework (or Sender ID Framework because MS likes to use this name, or DomainKeys by Yahoo). Efficiency of SPF largely depends on anti-spam measures at the RECIPIENT's server. If your record is good and the recipients Mail Transport Agent (POP server) reads SPF records before accepting mail, then spoofed addresses are rejected by the recipients server.
In short, you mention which computers on the Internet are allowed to send mail on your domain's behalf on the DNS records. POP servers while accepting mail, read that information, if the sender does not belong to one of those authorized by you, rejects the mail (or sends to a "blackhole").
It has a downside that if you run a mailserver from a dynamic internet IP address for some reason, chances are your mail will be rejected too.
To use SPF feature properly, you need full control on your DNS records. eUKhost gives you full access to the DNS records of your domain
. Looking forward to assist you for a smooth transition from your current host.
Let us know your domain and we'll see if you have an SPF records or not.
Regds
IJ