Round-robin mail & DNS question

Hello,

We are thinking of implementing round-robin mail servers using DNS, but are
unsure about what would happen if one of the mail servers were to go down. 
Let's say I have the following setup in the zone record:

@   IN   MX   10   mail.domain.com.
@   IN   MX   20   server1.domain.com.
@   IN   MX   30   server2.domain.com.
mail   IN   A   10.10.10.1
mail   IN   A   10.10.10.2

Note: 'server1' and 'server2' correspond to the '10.10.10.1' and
'10.10.10.2' IP's, respectively.

So, we understand that when a lookup is done on 'mail.domain.com', it will
round-robin between the two IP addresses.  We also understand that
normally, when the primary MX server becomes unavailable, it will go to the
secondary MX host.  This is for mail delivery, anyway.  But what happens on
the client side?  Let's say:

  - 'server1' (10.10.10.1) goes down
  - a client checks their mail, and on this instance, DNS grabs '10.10.10.1'
as the ip for 'mail.domain.com'

Will approximately 50% of the mail check requests fail, or will the weighted
MX records kick in here?  If so, then I guess we would need to look at
software that does two-way IP address take-over (i.e., if one server goes
down, the other takes over it's IP address)

Anyway, thank you, in advance, for your help and advice.

Alan Murrell <swasak@xxxxxxxxxxx>