DNS and Exchange

When installing SBS it is inportant to name the domain the same as the email domain name but with the .local correct? Or can I name the domain a shorter name if it is too long.
www.onelongname.com vs www.onelong.com
can I have:
Domain name: onelong.local
and still use www.onelongname.com for the email in exchange?

I understand that I will have to give the email hosting company or the ISP my static IP for a MX record correct?

and if I don’t have a static IP I can setup a free Dyn DNS service and then give them that domain name for email settings. is that correct?