hi.. to every1..

i'm having trouble with sendmail (the one that came with Red Hat Linux 8.0)

i have build 2 Red hat Linux Servers (8.0),
server 1: i configured mail server with a free domain, DYNDNS.ORG
everything seem's to B working fine.. my users are sending and receiving mail.. but the problem comes when one user, any user.. try's to send email to a specific domain.. actually 2 domains..

userX@domain.com and
userX@domain.gob.mx

the mail is return!!.. well, actually, i think it never goes anywhere.. it stays in the server.. in /etc/mail/mqueue
and it comes 'back' with the error: 451 Name Server Timeout

So.. i buil't another server.. with my own domain.. configured DNS and sendmail..
and i'm having the same problem.. well, i'm testing right now.. but i sent the email s again..
and one came back.. with the same error.. Name server timeout..
the other one.. still hasn;t come back.. and it's not in the /etc/mail/mqueue..
WAIT!!.. i think the folder is /var/spool/mqueue!! upss''

BUT WAIT!!, if i send the email 's from yahoo or hotmail.. they do get to their destination.. help!!!..

i have read.. that sendmail in red hat 8. has many troubles.. i still don't know how to update sendmail.. but first i wanna see what u think..

help pliz..