1> I have my domain's MX record pointed at a mailserver, using an IP
address.

No you haven't. The data portion of an "MX" resource record comprises a
domain name. It may have four all-digit labels, but it is _still_ a
domain name. It is _always_ a domain name.

And, of course, since it is a domain name, whether SMTP Relay clients
around the world can find the IP addresses of your SMTP Relay servers
depends from whether they can map that domain name to an IP address.

With some SMTP Relay clients, you'll be lucky. They'll call the DNS
Client library's normal name->address mapping routine to map the domain
name to an IP address, and either special-case code in the DNS Client
library (which some DNS Client libraries have) or special-case code in
the resolving proxy DNS server that the DNS Client queries (which some
DNS server softwares, such as Dan Bernstein's "dnscache", have) will
recognise a domain name that comprises four all-digit labels, and return
the appropriate IP address directly, short-circuiting normal query
resolution.

With other SMTP Relay clients, you'll be unlucky. They'll either have
their own hand-rolled DNS Client, without special case code, that they
use instead of the system-supplied one; or they'll call the DNS Client
library in the normal manner but neither their DNS Client library nor
their proxy DNS server have special-case code. In both cases, they'll
end up looking up the domain name in the public DNS database in the
normal fashion, and, of course, such domain names don't exist in the
public DNS database.

RFC 1537 recommends that in some fashion, either by DNS Client libraries
or proxy DNS servers having special-case code, queries for such domain
names be prevented from "leaking" out to the rest of Internet. However,
it is to cope with systems that _do not_ follow this recommendation that
one is discouraged from using four-label all-digit domain names in "MX"
resource records.