MX records

[systemec]

Hello,

We use several mailservers which are specified in the DNS records naturally,
for example:
      MX   10    smtp1.systemec.nl.
      MX   20    fallback.nl.uu.net.

Since we use more mailservers there are more MX records:
    MX   10    smtp2.systemec.nl.

    MX   10    smtp3.systemec.nl.

etc.etc.

We always use the same server for MX 20 (fallback.nl.uu.net.)
I assume then it is not necessary to specify each MX 20 record for the
various mailservers?

And if the above statement is true, what if I want to specify different MX20
records for the various mailservers ?

In other words, in what way is the DNS record being  processed to find the
necessary MX records.

Many thanks,

Marco Teelen