Using the unix dig
command, I can get the following MX record:
$ dig +nocmd gmail.com MX +noall +answer gmail.com. 1868 IN MX 20 alt2.gmail-smtp-in.l.google.com. gmail.com. 1868 IN MX 40 alt4.gmail-smtp-in.l.google.com. gmail.com. 1868 IN MX 5 gmail-smtp-in.l.google.com. gmail.com. 1868 IN MX 10 alt1.gmail-smtp-in.l.google.com. gmail.com. 1868 IN MX 30 alt3.gmail-smtp-in.l.google.com.
My question is, what does the fifth column (20, 40, 5...) mean?