validity of MX syntax?

[aklist_bind at enigmedia.com]

Hi All: is valid/OK syntax to apply a TTL directly to an MX record for an 
external host, like a mail-forwarder, for example:

$TTL 12h
@ IN SOA ns.mydomain.com. hostmaster.mydomain.com. (
 2006122102 ; serial
 12h ; refresh
 1h ; retry
 1w ; expire
 1h ) ;  neg cache
;
 NS ns.mydomain.com.
 NS ns1.mydomain.com.
;
 3600 MX 10 mailhost1.com.
 3600 MX 20 mailhost2.com.
 3600 MX 30 mailhost3.com.
 3600 MX 40 mailhost5.com.
 MX 50 mail
;
 TXT "v=spf1 mx ~all"
;
 A 123.123.123.123
;
mail 3600 A 123.123.123.124
www A 123.123.123.123

TIA