Adding an MX record

[#1]

I'm trying to add an MX record for an SMTP server hosted externally to plusnet, my domain is configured as follows:
subdomain  a x.x.x.166 
www a x.x.x.165 
 
mx 20 mx.last.plus.net. 
mx 25 subdomain.mydomain.co.uk. 
When I do an MX lookup on the domain I get:
Server:  ns1.force9.net
Address:  195.166.128.16
mydomain.co.uk MX preference = 10, mail exchanger = mx-ironport.core.plus.net
mydomain.co.uk MX preference = 20, mail exchanger = mx-ironport.last.plus.net
mydomain.co.uk nameserver = ns1.force9.net
mydomain.co.uk nameserver = ns2.force9.net
mx-ironport.core.plus.net      internet address = 84.92.1.97
mx-ironport.core.plus.net      internet address = 84.92.1.65
mx-ironport.last.plus.net      internet address = 84.92.1.97
mx-ironport.last.plus.net      internet address = 84.92.1.65
ns1.force9.net  internet address = 195.166.128.16
ns2.force9.net  internet address = 195.166.128.17
Do I need to change the email setting for the domain from the default to "I wish to specify DNS records for mail to this domain" for the change to have any effect, and if so, how can I keep the existing POP3 mail working (the external SMTP server is for sending only)? I tried specifying mx.core.plus.net and mx.last.plus.net as the the highest priority mx servers but this just made mail undeliverable to the domain.
Cheers,
Simon

[#2]

Hi Simon,

Quote from: shlawrence

Do I need to change the email setting for the domain from the default to "I wish to specify DNS records for mail to this domain" for the change to have any effect...

Yes.

Quote

...and if so, how can I keep the existing POP3 mail working

Errr, you can't. Choosing the option to specify your own mail exchangers assumes that you're using your own servers (or a third party's) to handle your *incoming* email.

Quote

...(the external SMTP server is for sending only)?

In which case you don't need to me messing with your mail records. As I alluded to above, your MX records are used for the receipt, and not the sending of email.
You should be able to send your email via whatever outbound server you desire assuming the owner of that server permits it.

[#3]

Thanks for the reply Bob, the reason I wanted to set up a low priority (high numbered) MX record for the outgoing server is it was failing validation on this by some other servers when trying to relay even though the reverse DNS, A record etc. was set up correctly; however I've decided it's a lot simpler to use a reputable external commercial SMTP service as an intermediate relay for the application as this will probably avoid a lot of "MTA reputation" based messing about.
In other news, I now seem to have terminally messed up the mail settings for this domain even though I've reverted them back to the defaults through the portal - that'll teach me!
Cheers,
Simon