Changing the domain name, impact on exchange server

yes,u can keep both domains in EMail
as smtp and active directory domains are two diff concepts
ad domain wont affect smtp domain

i hope you are talking about Email domain and not Active Directory domain.

Yes exchange can manage as many emails domains as you want simply add them to the list of accepted domains

However renaming an active directory domain will break your exchange

Thank you for the response. No, I think I will keep the internal domain, what we are changing is our website domain and the email as well. So, I just need to add another domain in exchange? is it that simple? Also, can the both domains (old and new) have the same MX record with my ISP?

Yes, u can have two domains having same MX record.
U have to inform ur ISP about the same.

Thanks for the response. How would Outlook handle two domains? two accounts?

Outlook can only be setup with one exchange account so the options are

1 account 2 aliases
2 accounts and 2 outlook profiles
2 accounts 1 outlook profile configured to open the mailbox of the 2 users

The users would be stamped with two email address,they will by default receive emails for both domains.
For sending primary email address will take precedence.
if u want to send using secondary account,use the from field and enter that email address.

The old domain will ventually go away, I only need it because we have many users that have been using it for years, once all their contacts have the new email I will eventualy get rid of it. Can I have outlook configured for the new email account and have exchange forward emails on te old address to the new address?

yes,u can.

no need to do that
just update the recipient policy to add an smtp address @newdomain.com and set it as primary

all users will still receive to both addresses but will send from the new one