mx priority question

[birch69x]

Here is the setup:
mx1.xxxx.com priority 10
mx2.xxxx.com priority 20

As I understand it, sendmail will try to use mx2.xxxx.com if
mx1.xxxx.com is down? Well, but sendmail just put the mail in queue
and tried mx1.xxxx.com later.
Have I misunderstood the mechanism or what have I done wrong?
Thanks.