Contents of dig mx mydomain.co.uk is this.
; <<>> DiG 9.3.4-P1.1 <<>> mx mydomain.co.uk
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28347
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;mydomain.co.uk. IN MX
;; ANSWER SECTION:
mydomain.co.uk. 86400 IN CNAME myserver.servehttp.com.
;; AUTHORITY SECTION:
servehttp.com. 60 IN SOA nf1.no-ip.com. hostmaster.no-ip. com. 2007134962 90 120 604800 60
;; Query time: 448 msec
;; SERVER: 192.168.2.1#53(192.168.2.1)
;; WHEN: Wed Nov 12 17:57:38 2008
;; MSG SIZE rcvd: 128
Also I have had a look into the problem with yahoo, rejecting the email because it is from an ip address
553 5.7.1 [BL21] Connections not accepted from IP addresses on Spamhaus PBL
In our continuing efforts to protect our users from unsolicited email, Yahoo! Mail does not accept SMTP connections from dynamic or residential IP addresses, as determined by the Spamhaus PBL. If you are seeing this error message, please do not retry sending your message using the same IP address. Instead, please contact your ISP for information on which SMTP server you are authorized to use for your outbound email.
If you think that your IP address has been listed in error, you or your email administrator should contact Spamhaus. Once your IP is de-listed by Spamhaus, Yahoo! Mail will automatically unblock your IP within 48 hours.
Have I set something up incorrectly or is it always sent via an IP address?
Just a note it works fine if I send to an email address that is not yahoo, but that is not very good for any clients.