domainx.local for Active directory, domainy.com for web and e-mail. How manage mx record with local exchange?

You don't need to make an internal MX record to route email.  The clients will connect directly with the exchange server.  You should have one MX record at whoever's hosting your public DNS pointing to the public interface of your exchange box (or spam filter proxy).

This should be a very easy change for you.  Once you change the MX record, provided the exchange and firewalls are set up right, the mail should start flowing in to your box instead of the ISP's.

you must make sure that the user accounts have mailboxes on the exchange server, and that their email addresses correspond to the ones that already exist with your current host.

Thanks for ur comment...but let me understand better.
My exchange local account is: tom@domainxx.local
My isp mail is : tom.smith@domainyy.com
If i route e-mail to my exchange instead of isp mail server, how exchange manage this situation?

you say
"you must make sure that the user accounts have mailboxes on the exchange server, and that their email addresses correspond to the ones that already exist with your current host."
so ii's enough i modify my exchange local username from "tom" to "tom.smith"?

After this, exchange starts to route internal email only locally?
(it has to route only locally an e-mail from tom.smith@domainxx.local   to fred.smith@domainxx.local, but route outside an e-mail from tom.smith@domainxx.local to otheremail@otherdomain.com)

What i'm no understanding if is exchange can coreectly manage domainxx.local and domainyy.com

a. You need to set accepted domain policy coz you have domail.local which exchange will undestand and you will have to add domain.com in accepted domain to Exchange will start accepting email id with user@domain.com
b. Create a reciepent policy so when new user is creaetd he has bolth email ID user@domain.local and user@domain.com and set user@domain.com as primary

Trying to closing circle...then i'll try in a test virtualized environment.
a) Migrate to an Isp for email and dns could provide me space and features (like Etrn...) better then actual isp (this step is forced by actual isp limits)
b) Pointing mx record to exchange public interface. At this time, exchange has to be set to accept domainyy.com, and all user of domainxx.local should have right "translation" for the user of domainyy.com by recipient policies
c) So exchange should be manage correctly inbox email (my goal is two mx record on my two adsl connection, third mx record on Isp Etrn service as backup). Having aliases with recipient policies, mail to tom.smith@domainyy.com goes to tom@domainxx.local.OK
And outgoing email? same benefit from recipient policies? Exchange let go out an e-mail from  tom@domainxx.local just like  tom.smith@domainyy.com?
d) publishing Owa, having Isp webmail like backup only for "outside" email and when my system is down
e) can i manage "delegates" in this situation?
Thanks again...
Nanoweb

a) I have static Ip on primary dsl and static ip on secondary dsl. I want to set two mx record for these, my isp mail server like third mx for backup (etrn not available from my isp.....  =:-(     ).

b)ok

c) which is the "best practices" approach, managing mx record? any info,articles, example please?

for exchange:
d1) setting an accepted domain policy (@.com)
for every internal adress:
d2) setting a recipient policy to get an alias (tom.smith@.com & tom@.local )
d3) setting the primary smtp address on tom.smith@.com

Hi,

c) which is the "best practices" approach, managing mx record? any info,articles, example please?
 It's easy to setup mx record. The only thing that you need to keep in mind is that the lowest preference mx record will be picked up first. Two mx record with same preference will respond in round robin fashion. The higher preference mx record if exist will only be picked up when lower preference record doesn't respond.

I have to try this teoric way...but people explained well and focus my needs.