There doesn't seem to be an authoritive answer to this that I can find (or my Google-fu isn't working), but this page here implies that if an A record points at IP address that is running an SMTP server, the sending server will talk with that IP address to attempt to deliver the mail.

Presumably, if MX records are present they supersede all A records.

Thanks for that.

I shall leave it for the time being and not worry about it unless emails stop arriving.

Am I right to assume the "@" record is email related?

@ in DNS records means root, i.e. no sub-domain. They can be used on all types of records (I believe), including A and CNAME although having a @ CNAME record prevents all other @ records working (again, I believe)

Since no-ip have gone down because of Microsoft being cunts I actually got my subdomains working dynamic properly now.

Ended up using the free nameservers at afraid.org and pointing things through them.

That's how I do mine as well.  And I use Tomato so the router actually notifies afraid.org when a change happens.  Very set it and forget it and I like things like that!

5 min cronjob on one a local server is what I've been using.

I do something similar to afraid.org with Linode and their DNS Manager and PHP SDK. Useful if you already have hosting with them.