Usually it would be the interface on the server or router or firewall that has your public ip address. A common way is to ping your external domain name. For example if your email address is yourname@yourcompany.com ping yourcompany.com - this should work internally as long as ICMP is allowed and your internal and external domain names are not the same.
Richard's approach is correct. It's usually best to query MX records from outside your own network - from an ADSL or dial-up connection for example. This will avoid any confusion if you are using the same namespace for internal and external DNS, but have the public and private zones on seperate servers. This is very common, as most companies have their public DNS zones managed by a third-party, for example, their ISP.
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